Partially skip test due to BZ 1163494

Issuing an HTTP GET request for a non-existent repository triggers an HTTP 500
error with this error message:

    NoMethodError: undefined method `[]' for nil:NilClass

This only occurs if a repository with the given ID once existed. Fetching a
repository that has never existed will trigger an HTTP 404 as usual with a
message like this:

    Couldn't find Katello::Repository with id=600

Updated test results:

    $ nosetests tests/foreman/api/test_multiple_paths.py:DoubleCheckTestCase -m 'test_delete_and_get.*Repository'
    .
    ----------------------------------------------------------------------
    Ran 1 test in 11.153s

    OK