Add rabbit_ha_queues option

In current, rabbit_ha_queues option is controled by rabbit_hosts
if conditional statement. When users want to set rabbit_ha_queues
to true and they don't use rabbit_hosts option, this logic will
not satisfy their needs.

This patch adds an $rabbit_ha_queues option, set it to undef by default
for forward compatbility.

Change-Id: Iaf6157f6b7acb148bd7da41421b4a3de6c4e2194
Closes-bug: 1361700
Merge "Add rabbit_ha_queues option"