this commit introduces a new '/federations'
endpoint to Magnum API, as well as its controllers,
entities and conductor handlers.
this corresponds to the first phase of the
federation-api spec. please refer to [1] for more
details.
[1] https://review.openstack.org/#/c/489609/
Change-Id: I662ac2d6ddec07b50712109541486fd26c5d21de
Partially-Implements: blueprint federation-api