Increase Resource test coverage
Increase Resource test coverage

There were a bunch of untested or partially tested branches, including
some that were not explicitly tested but only covered by their usage
elsewhere. This introduces a test class for the MutableMapping
functionality, tests the *_by_id methods and their branches, and several
other expansions on branch coverage. Combined with two other smaller
patches, this brings coverage to 100%.

Change-Id: Ie94662c90c98ea17ba3a927953a3804dc1c0f8c5

dtroyer/python-openstacksdk,stackforge/python-openstacksdk,dtroyer/python-openstacksdk,stackforge/python-openstacksdk,openstack/python-openstacksdk,dudymas/python-openstacksdk,briancurtin/python-openstacksdk,dudymas/python-openstacksdk,briancurtin/python-openstacksdk,mtougeron/python-openstacksdk,openstack/python-openstacksdk,mtougeron/python-openstacksdk