It is possible that a user might want to use environment parameters
(namespace or server) in their parameters. This would not work with
ksonnet for two reasons:

1. ksonnet didn't provide external variables with those settings
1. params.libsonnet was evaluated as an object

This change adds support for these two items.

```
bash-3.2$ ks param list
COMPONENT PARAM         VALUE
========= =====         =====
ds        arr           [1, 2, 3, 4]
ds        containerPort 80
ds        image         'gcr.io/heptio-images/ks-guestbook-demo:0.1'
ds        name          'ds'
ds        namespace     ''
ds        obj           { a: 'b' }
ds        replicas      1
ds        server        ''
ds        servicePort   80
ds        type          'ClusterIP'
bash-3.2$ ks param list --env default
COMPONENT PARAM         VALUE
========= =====         =====
ds        arr           [1, 2, 3, 4]
ds        containerPort 80
ds        image         'gcr.io/heptio-images/ks-guestbook-demo:0.1'
ds        name          'ds'
ds        namespace     'default'
ds        obj           { a: 'b' }
ds        replicas      1
ds        server        'https://localhost:6443'
ds        servicePort   80
ds        type          'ClusterIP'
```

An example parameters file that produces the preceeding output:

```js
local env = std.extVar("__ksonnet/environments");

{
  global: {
  },
  components: {
    ds: {
      containerPort: 80,
      image: "gcr.io/heptio-images/ks-guestbook-demo:0.1",
      name: "ds",
      replicas: 1,
      servicePort: 80,
      type: "ClusterIP",
      namespace: env.namespace,
      server: env.server,
      obj: {a:"b"},
      arr: [1,2,3,4],
    },
  },
}
```

* note: params files aren't currently generated with the `env` local.

Signed-off-by: bryanl <bryanliles@gmail.com>