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.
`LifecycleEnvironment.prior` now defaults to `None`.