repository tests using apis
Do not check content-type of HTTP 204 messages

Checking the content-type of an HTTP 204 message does not make sense, as HTTP
204 messages "MUST NOT include a message-body". Make the test suite reflect this
fact.
Merge pull request #1274 from sghai/repo-api

repository tests using apis
Merge pull request #1286 from Ichimonji10/http-204

Do not check content-type of HTTP 204 messages
Add API tests for host collections

Update the `HostCollection` class definition: add mixins for creation, reading
and deletion. Remove the `system_uuids` attribute: no information of the sort is
provided when reading a host collection, and when creating or updating a host
collection, the attribute seems to conflict with the `system_ids` attribute.

Thoroughly test the relevant API endpoints.
Test ActivationKey/HostCollection associations

Add a test that associates an activation key with zero, one, two and then zero
host collections. Remove a test which currently attempts to test the same thing
but is failing for some unknown reason.