Refactor mDNS packet finalization
Refactor mDNS packet finalization

mDNS packet finalization, i.e. writing out the packet header and
adding TSIG was duplicated. Refactor this into a smaller reusable
method.

Change-Id: I3fffc2b259044866933987aaed4c407a7febdeba

ramsateesh/designate,ramsateesh/designate,grahamhayes/designate,grahamhayes/designate,openstack/designate,ramsateesh/designate,ionrock/designate,grahamhayes/designate,muraliselva10/designate,tonyli71/designate,cneill/designate-testing,cneill/designate-testing,muraliselva10/designate,muraliselva10/designate,tonyli71/designate,cneill/designate-testing,ionrock/designate,openstack/designate,ionrock/designate,openstack/designate,tonyli71/designate