Add dhcp_domain option to dhcp_agent.ini

This will allow you to set the domain dnsmasq hands out
to instances.
From: http://docs.openstack.org/trunk/openstack-network/admin/content/dhcp_agent_ini.html
dhcp_domain=openstacklocal  (StrOpt) Domain to use for building the host names.

Defaults to "openstacklocal"

Change-Id: Id963b7a6eeb7d57a5a0797d4e1e2981b8fd9e7a1