this commit introduces a new `Federation` table to
Magnum database, as well as the necessary DB layer
APIs to access and manage it.
this belongs to the first phase of the implementation
of the federation api. check [1] for more details.
[1] https://review.openstack.org/#/c/489609/
Change-Id: Ie8a68cd3198c8fc7930069fd2e55f1cad55b6c9b
Partially-Implements: blueprint federation-api