Ensure entries in dnsmasq belong to a subnet using DHCP

In certain configurations, Neutron calculates SLAAC addresses for IPv6
subnets and adds them to the fixed_ips field of a port. Since those
subnets are not being managed by DHCP, do not add those fixed_ip entries
to the host file.

Fixes-bug: #1316190
Related-bug: #1257446

Change-Id: I77dd55063296990c9df385f331f5de5d42402786