Adds --addn-hosts to the dnsmasq arg list

When using multi_host=True and update_dns_entries=True, dns records from
other compute hosts were not able to be resolved.  The --addn-host arg was
constructed but not appended to the cmd list.

Change-Id: I9cafb59256e92bdad0df3e9166e0afd079ae0c09
Fixes: bug #1170529
Merge "Adds --addn-hosts to the dnsmasq arg list"