Improve test for updating repository content type

A repository's content type cannot be updated. Make a test which updates a
repository's content type explicitly check to make sure that its content type is
not updated.

    $ nosetests tests/foreman/api/test_repository.py -m test_update
    .............
    ----------------------------------------------------------------------
    Ran 13 tests in 25.713s

    OK

This should have been implemented in fb5f58094d7b64f7bd355ce5bd1c844f5766ab0b.
Make repository searches more robust

Issuing HTTP GET requests so /katello/api/v2/repositories yields inconsistent
results. The exact same search can return a correct list of results or an empty
list. Compensate for this by adding some polling logic to
`Repository.fetch_repoid`. This produces more consistent results:

    $ nosetests tests/foreman/api/test_repository.py -m test_redhat_sync_1
    .
    ----------------------------------------------------------------------
    Ran 1 test in 51.665s

    OK
Merge pull request #1799 from Ichimonji10/test-repo-2

Improve test for updating repository content type
Merge pull request #1800 from Ichimonji10/repo-search

Make repository searches more robust