Add API tests for LifecycleEnvironment and GPGKey

Add several API tests which target the following URLs:

* `katello/api/v2/gpg_keys`
* `katello/api/v2/environments`

Many tests were already in place for GPG keys. However, there was no test for
issuing a GET to `katello/api/v2/gpg_keys`. This commit fixes that. In contrast,
there were no tests for lifecycle environments, and this commit adds a basic set
of HTTP POST and GET requests for lifecycle environments.
Will do. That makes sense, at least for now. Funnily enough, I did try creating a couple LifecycleEnvironment objects, and was able to do so via this call:

```python
LifecycleEnvironment().create(fields={'prior': 2})
```

However, this fails:

```python
LifecycleEnvironment(prior=2).create()
```

This is because the API code assumes that all foreign keys will have `_id` appended to the end of their name. Thus, that second call sends a `prior_id` of `2` to the server. The server does not recognize such an argument, which is horribly inconsistent of the server. The first call succeeds, because `fields={...}` inserts data into the call after name mangling is performed.