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
Make `ContentView` readable, trim down smoke test

Make `ContentView` inherit from `EntityReadMixin`. use this mixin to trim down
module `tests.foreman.smoke.test_api_smoke`: replace many usages of `client.get`
with `ContentView.read_json`.