BigSwitch: Improves server manager UT coverage

Improves the unit test coverage for the Big Switch
server manager module (100%). Also reorganizes the
capabilities test to avoid duplicating a lot of
router tests that are already covered.

Closes-Bug: #1289027
Change-Id: Ib51e8160f8d95686e86430b0a9c9f54deeda0e84
Merge "BigSwitch: Improves server manager UT coverage"