Enable magnumclient OSC plugin support.
This patch set does the following:
- Establishes OSC plugin structure for commands and unit tests
- Creates the OSC plugin
- Adds OSC to test-requirements.txt
This fixes are mainly copied from python-neutronclient.
Partial-Implements: blueprint openstackclient-support
Change-Id: I8f7ccde7d1417416ada5fe5e88832db7520efa6f