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

`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`.

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.

Fixes #108.

For example, 'apply <env' currently operates as a no-op. With the
introduction of simple environments in PR #131, 'apply <env>' should
perform basic validation such that:

1. The user has added the environment that is being deployed against to
their Ksonnet project.

2. The URI in the environment's spec file that the user wishes to deploy
to should correspond to at least one cluster location as listed in
kubeconfig.

If either of those conditions are not satisfied, the kubecfg user will
receive the corresponding error.

In addition, this commit will set the kubectl --cluster flag to point at
the cluster listed by the environment URI.

Fixes #139.

'env set <name>' sets environment fields such as the name, and cluster
URI. It currently accepts the flags '--name' and '--uri'. Changing the
name of an environment will also update the directory structure in
'environments'.

'env rm <env-name>' deletes an environment within a ksonnet project.
This is the same as removing the <env-name> environment directory and
all files contained.  Empty parent directories will also be removed.

'env list' will list all environments within a Ksonnet project. Each
environment will be pretty-printed with it's name and cluster URI
location.

'env add <env-name> <env-uri>' will create a new environment within a
ksonnet project, by generating a new directory, 'env-name', within the
'envs' directory. Each environment will contain environment-specfic
files. Notably, a new environment-specific file is 'spec.json'.
'spec.json' currently only contains the 'env-uri' of the Kubernetes
cluster located at the added environment.

Below is an example directory structure for the environment
'us-west/staging':

app-name/
  .gitignore           Default .gitignore; can customize VCS
  .ksonnet/            Metadata for ksonnet
  environments/        Env specs (defaults: dev, test, prod)
    default/           [Default generated environment.]
    us-west/           [Example of user-generated env]
      staging/
        k.libsonnet
        k8s.libsonnet
        swagger.json
        spec.json      [This will contain the uri of the environment]
  components/          Top-level Kubernetes objects defining application
  lib/                 user-written .libsonnet files
  vendor/              mixin libraries, prototypes