Validate the existence of DHCP server IP for vsd managed subnet
Validate the existence of DHCP server IP for vsd managed subnet

For vsd managed l2 subnet:
    Check DHCP server ip(gateway/IPv6Gateway) is empty for subnet
with disable dhcp. IP4 l2domain with dhcp enabled but null dhcp ip
is blocked on vsd, but not for ipv6. For IPv6 case, an invalid ip is
gave(vsd bug ticket VSD-35558).
    Check DHCP server ip(gateway/IPv6Gateway) is not empty for subnet
with enable dhcp

Change-Id: I76b0ff1a065143f2310cc39a4ca7dd3a1be4f3f5
Depends-On: Iffab7c5149772d8fcd9e936b9f309cfc7e29d295

nuagenetworks/nuage-openstack-neutron,nuagenetworks/nuage-openstack-neutron