Do not try to update a repository's content_type

From IRC:

    <ichimonji10> Hey all. Is it possible to update the content_type of a
    repository?
    <jsherrill> ichimonji10: its not
    <jsherrill> you have to delete and re-create

This fixes a failing Jenkins test. Updated test output:

    $ nosetests tests/foreman/api/test_repository.py -m test_update
    ....S........
    ----------------------------------------------------------------------
    Ran 13 tests in 29.033s

    OK (SKIP=1)
Merge pull request #1797 from Ichimonji10/repo-update

Do not try to update a repository's content_type
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.
Merge pull request #1799 from Ichimonji10/test-repo-2

Improve test for updating repository content type