Skip known-failing test (see BZ 1154803)

Issuing a PUT request to the /users/:id API path will fail if the request
includes the `auth_source_id` parameter. The API doc states that this
parameter is acceptable, but the application returns an HTTP 500 error with this
message:

    auth_source_id is not allowed as nested parameter for users. Allowed
    parameters are auth_source_ldap_id, role_id, location_id, organization_id,
    usergroup_id

Test results:

    $ nosetests tests/foreman/api/test_multiple_paths.py -m 'test_put_status_code_.*_User'
    S
    ----------------------------------------------------------------------
    Ran 1 test in 0.636s

    OK (SKIP=1)