Updates external DNS service if IP address changes
Updates external DNS service if IP address changes

Previously the external DNS service was only notified when dns_name
was updated in a port, but it should also be alerted when the IP
address changes. This patch propogates the changes invoked in a port
update to the external DNS service when either the dns_name or the IP
address of a port changes.

Change-Id: I319f7a3265caaee5edaab19695a99d84e8e9a8ab
Closes-Bug: 1554207

eayunstack/neutron,openstack/neutron,sebrandon1/neutron,mahak/neutron,openstack/neutron,noironetworks/neutron,klmitch/neutron,igor-toga/local-snat,huntxu/neutron,MaximNevrov/neutron,cloudbase/neutron,wolverineav/neutron,wolverineav/neutron,bigswitch/neutron,noironetworks/neutron,bigswitch/neutron,eayunstack/neutron,openstack/neutron,cloudbase/neutron,mahak/neutron,klmitch/neutron,huntxu/neutron,igor-toga/local-snat,MaximNevrov/neutron,mahak/neutron,sebrandon1/neutron