Pool Manager - BIND9 Pool Backend Driver Fixes

BIND9 Global Option Changes:
- Provide default ([127.0.0.1:5354]) for masters option
(master is now of the form <ip-address>:<port>)

BIND9 Server Specific Option Changes:
- Provide default (127.0.0.1) for host option

Additional Changes:
- Update designate.conf.sample
- Retrieve backend option values when backend driver is initialized to improve
performance
- Provide minimum validation of masters backend option

Change-Id: Ic9ffa51b2ee6c273e246a3dd7cc96cf7180d8f95
Partially-implements: blueprint server-pools-service