Adds support for contextNames in tk env subcommands

[Nashluffy]

This compliments #674 and probably should have been in there to begin with.

This PR adds support for contextNames using tk env add and tk env set commands. It leverages the Connect method to check validity of the resulting config.

Below are a few examples, including cases that would throw errors

(~/Code) Nash $ mkdir example-tk && cd example-tk
(~/Code/example-tk) Nash $ tk init
GET https://github.com/jsonnet-libs/k8s-libsonnet/archive/1942ec15993b5a273569bb9a59d522b01522d0db.tar.gz 200
GET https://github.com/grafana/jsonnet-libs/archive/73396f2821466dd8f91181e7ee4513e0a8ea45b3.tar.gz 200
Directory structure set up! Remember to configure the API endpoint:
`tk env set environments/default --server=https://127.0.0.1:6443`


(~/Code/example-tk) Nash $ tk env set environments/default --context-name "devwat-us-south.*" --server localhost --namespace default
updated spec.apiServer (`` -> `localhost`)
updated spec.contextNames (`[]` -> `[devwat-us-south.*]`)
Error: Your Environment's spec.json seems incomplete:
  * spec.apiServer|spec.contextNames: These fields are mutually exclusive, please only specify one.

Please see https://tanka.dev/config for reference

(~/Code/example-tk) Nash $ tk env set environments/default --context-name "devwat-us-south.*" --namespace default
updated spec.contextNames (`[]` -> `[devwat-us-south.*]`)
(~/Code/example-tk) Nash $ cat environments/default/spec.json
{
  "apiVersion": "tanka.dev/v1alpha1",
  "kind": "Environment",
  "metadata": {
    "name": "environments/default",
    "namespace": "environments/default/main.jsonnet"
  },
  "spec": {
    "apiServer": "",
    "contextNames": [
      "devwat-us-south.*"
    ],
    "namespace": "default",
    "resourceDefaults": {},
    "expectVersions": {}
  }
}

(~/Code/example-tk) Nash $ tk env set environments/default --server localhost --namespace default
updated spec.apiServer (`` -> `localhost`)
Error: Your Environment's spec.json seems incomplete:
  * spec.apiServer|spec.contextNames: These fields are mutually exclusive, please only specify one.

Please see https://tanka.dev/config for reference

(~/Code/example-tk) Nash $ tk env add example --context-name "devwat-us-south.*"
(~/Code/example-tk) Nash $ cat example/spec.json
{
  "apiVersion": "tanka.dev/v1alpha1",
  "kind": "Environment",
  "metadata": {
    "name": "example"
  },
  "spec": {
    "apiServer": "",
    "contextNames": [
      "devwat-us-south.*"
    ],
    "namespace": "default",
    "resourceDefaults": {},
    "expectVersions": {}
  }
}
(~/Code/example-tk) Nash $

[malcolmholmes]

There is good reason why context name is not used in spec.json: it can vary across installations, meaning different users get differing results.

What could work would be allowing a tk env add command that uses a context name, but writes the apiServer URL into the spec.json instead. That way, initial user experience is improved (I can specify the cluster using a term I'm familiar with) yet consistency is maintained (every user references the same api server).

[Nashluffy]

@malcolmholmes support for context names in spec.json was added in v0.21.0 release of Tanka (introduced in this pull request). This is just adding the corresponding functionality to tk env commands.

[malcolmholmes]

@Nashluffy I'm clearly out of date!

[julienduchesne]

LGTM! Makes sense that if it's supported in the spec.json, it should be in these commands