Support `diff`ing between two environments

Transition to pure Go implementation of Jsonnet

This change enables the user to diff between two environments that are
either local or remote.

i.e.,

`kubecfg diff local:dev local:prod` will diff between the expanded
templates for each environment on disk.

`kubecfg diff remote:dev remote:prod` will diff between two remote
environment clusters. It does this by first expanding the component
templates of each environment. Then, the live objects are fetched from
each of the clusters and the diff is performed against the live objects.

`kubecfg diff local:dev remote:prod` is also an option. This will diff
between the expanded templates for 'dev' on disk and the live objects
on 'prod's server.

 Set env namespace to default when namespace not provided

For now, client-go bindings will be removed from the diff command.

Doing this because when diffing between multiple remote environments, it
becomes ambiguous which environment cluster the flags should belong to.

`ks env set foo --namespace=""` and `ks env add foo` will both set the
namespace to 'default' in the environment's spec.json file.

 Init env metadata from provided context

`ks env add myenv --context=dev` will initialize the environment URI
and namespace based on the context `dev`, as specfied in the
kubeconfig file. The same applies for `ks init foo --context=dev`.

If both the context and uri argument is not provided, the current
context is used.

This change will also remove uri as a mandatory arg in `ks env add`.
The uri will be moved to a flag. It cannot be used at the same time as
`context`.

[docs] update readme

Signed-off-by: Jessica Yao <jessica@heptio.com>

Remove link to releases page

ksonnet.next isn't released yet. This message remained as a port from
the original README file in the kubecfg project.

Rename binary to 'ks';  rename 'kubecfg' references to 'ksonnet'

Introduce namespace to environments

Environments currently have the concept of a server URI, but it is
ambiguous which cluster namespace to use.

This commit will introduce the concept of namespaces to the env
commands.

For example,
`kubecfg env add staging http://mock-staging-uri \
 --namespace=staging-namespace`
`kubecfg env set staging --namespace=staging-namespace`

The default environment will use the namespace of the default context.

This commit will also update commands that take the <env> arg such as
`apply` to make use of the env namespace, if specified.

Environment base.libsonnet and overrides

Commands that take `env` as a param currently expand all files in the
`components` directory. This is no longer necessary with the
introduction of `base.libsonnet` and the per-environment override
`<env>.jsonnet` file.

This commit will simply expand the single `<env>.jsonnet` file (which
will implicitly expand all component files). The case of running
`ksonnet apply default`, is equivalent to running `ksonnet apply -f
environments/default/default.jsonnet`.

This commit will generate an "<env-name>.jsonnet" file when an
environment is created.

For example, the default environment would have the following tree
structure:
├── environments
│   ├── base.libsonnet
│   └── default
│       ├── .metadata
│       │   ├── k.libsonnet
│       │   ├── k8s.libsonnet
│       │   └── swagger.json
│       ├── default.jsonnet
│       └── spec.json

The goal of this file is to allow users to extend on base.libsonnet on a
per-environment basis to allow for custom overrides, such as replica
count.

base.libsonnet is a generated file that exists at the root of the
environments directory. This file is generated for all ksonnet projects.
The main goal of this file is to import all components in the components
directory, so that environments are able to easily extend / override any
one of these components in a modular structure.

This change simply hides the environment metadata details that we do not
want users directly modifying into a .metadata folder.

Remove unnecessary param from defaultNamespace()

The `clientConfig` param currently being passed is not needed, because
it exists as a package level var in root. It also makes little sense to
pass a custom `clientConfig` because if `overrides.Context.Namespace` is
populated, the namespace that is returned is configured as an override
in the package level `clientConfig` and not the `clientConfig` in the
param.

Remove client-go flags from commands that don't use

Add arg length constraint to commands

Construct base components Obj

In order to support environment heirarchy, we need to be able to
reference all components. This commit will implement component imports
as k-v pairs ex: foo: "import/foo.jsonnet" as a Jsonnet object. This
jsonnet object will then be assigned as an ExtCode so that it can be
referenced by a base.libsonnet file which environments can build /
override upon.

Fix 'env set' where --name flag is not specified

There is a bug where if --name is not passed during env set, an error is
thrown due to failed validation checking on an empty name. This commit
fixes this bug.

Add combination deployment/service system prototype

Implement 'prototype preview'