When a Subnet's ipv6_ra_mode is None, do not start up dnsmasq

Change-Id: I2d444c2089e87fff7092d5a7bc18b247f126ad0e
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.

Implements blueprint ipv6-provider-nets-slaac

Change-Id: I77dd55063296990c9df385f331f5de5d42402786