DevStack: Support devstack with bind9 backend on Fedora

This adds support for running devstack on Fedora 20 with
support for the bind9 backend in designate.
Adds the $STACK_USER to the bind9 daemon group, and uses sg to
start designate-central, because designate-central needs read/write
access to the bind9 db/zone files.

Closes-Bug: #1325655
Change-Id: Ib2a2d619dbd41234017d45871e8b11aa093df686