Add support for DNS zone fusion

Add support for DNS zone fusion

This submission address the "fusion" between DNS service zones
between DCAE and rest of ONAP R1.  In ONAP R1 each ONAP deployment
runs its own private DNS server; DCAE use Designate DNS as a Service.
The VMs on both sides need to be able to communicate with each other
by hostnames.  This submission modifies the ONAP private DNS server
so it uses the DNS server behind the Designate as the forwarder.  For
ONAP->DCAE communication, host names will be resolved by Designate DNS
server; for DCAE->ONAP communication, since DCAE VMs also use the ONAP
private DNS server as first name server, it is not a problem.

Issue-Id: DCAEGEN2-187
Change-Id: Ia54cd41f33b446d406a69868598ae1a8c0cdea8d
Signed-off-by: Lusheng Ji <lji@research.att.com>