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
(cherry picked from commit 3922da72b2569976f25183c476cedcc17d32fa13)
Merge "Adds --addn-hosts to the dnsmasq arg list" into stable/grizzly
fix non reporting of failures with floating IP assignment

It seems that commit essex-1-1003-g9728ae5
inadvertently removed a 'raise', thus hiding
all processutils.ProcessExecutionError exceptions
apart from the one indicating "Interface not found".

Fixes bug: 1178658
Change-Id: I2bcf8c3a19ebebc81a38ce6bf7b54065d62834c0
(cherry picked from commit 175584f0afef308e1021bb4daf49cbf222a61a40)