Add test that ensures subnet is usable after update
Add test that ensures subnet is usable after update

Adds a simple test to make sure a port can be created
on a subnet after disabling DHCP.

Related-Bug: #1573443
Change-Id: I94f1a59e52b109984904fd9e56e3c1a3f1c270fe

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