BIND9 Plugin Cleanup

General cleanup of the bind9 plugin. Adds rndc addzone/delzone
functionality, which speeds up the plugin by allowing for dynamic
syncing of zones. Removes management of central bind configuration
from designate copying bind's statement of zones over. Also
updates some documentation to reflect the change.

Change-Id: I474ac1c2f0f1b5d8027862ac372bad653cb5d7ea
Merge "BIND9 Plugin Cleanup"