Specify the dns_domain value in dhcp_agent.ini configuration
file in order to indicate the dns search domain which should
be advertised by the dnsmasq DHCP server.
Note, for neutron-openvswitch - this only takes effect when
the enable-local-dhcp-and-metadata flag is set to true.
Change-Id: If3529cf32a6e10d44c86423151cdacdad50445f8
Implements: blueprint charms-internal-dns