Tweak smoke test "get_api_links"

Make the errors produced by the test more useful. For example, this comparison:

    assertEqual(frozenset(foo.keys()), frozenset(bar.keys()))

produces much nicer error messages than this comparison:

    for k in foo.keys():
        self.assertIn(k, bar)

Rename several variables for the sake of clarity. For example, use this
assertion:

    self.assertEqual(
        frozenset(api_paths[group]),
        frozenset(API_PATHS[group])
    )

Instead of this assertion:

    self.assertEqual(frozenset(paths), frozenset(API_PATHS[category]))
Update robottelo's list of API paths

Several new API paths have been added.

* @cfouant can vouch for the `activation_keys` paths.
* jsherrill can vouch for the new `errata` path.
* mmccune can vouch for the new `mail_notifications` paths.
* partha can vouch for the `docker_images` and `docker_tags` paths.

There is a bug in apipie, so these paths differ from what the server currently
returns. This bug is described in BZ 1166875. Make the test code accomodate this
bug.
Merge pull request #1742 from Ichimonji10/new-api-paths

Update smoke tests for API paths