Make EntityDeleteMixin.delete deal with HTTP 204s

An HTTP 204 response indicates that a request has been processed but no content
is being returned. When this happens, any attempt to decode the JSON in a
response will fail, because no content is available to decode that content. Make
`EntityDeleteMixin.delete` deal with this case. This fixes a test failure.
Updated test results:

    $ nosetests tests/foreman/api/test_multiple_paths.py:DoubleCheckTestCase -m 1_ActivationKey
    .S.
    ----------------------------------------------------------------------
    Ran 3 tests in 31.344s

    OK (SKIP=1)
Merge pull request #1751 from Ichimonji10/delete-204

Make EntityDeleteMixin.delete deal with HTTP 204s
Merge pull request #1752 from Ichimonji10/unused-method

Drop unused method Organization.fetch_rhproduct_id
Fixed:test_remove_filter:: updated locator
Fixed: activation_key tests as per the changes in api code for CV promote/publish
Fixed locators
Fixed: Failing GPGKey tests while searching the key
Fixed the host-collection update failing in jenkins
Merge pull request #1754 from sghai/fix-locators

Fixed few UI tests
Merge pull request #1753 from tkolhar/test_cli

Host-collection update command  is failing becoz of the BZ: 1171669
WIP: EntitySearchMixin