Pool Manager Service Changes

- Added the Pool Manager cache (including test cases)
- Added the pool_manager_status table (including test cases)
- Added the PoolManagerStatus object
- Added the PoolServer object
- Added the BackendOption object
- Added support for global and server specific backend configurations
- Added a backend driver for pool manager (BIND9)
- Stubbed out the pool manager service (including periodic_sync)

Change-Id: I3d94e5df663d2938b0e6017fe43c28c71f7a29a0
Partially-implements: blueprint server-pools-service
Merge "Pool Manager Service Changes"