Allow to use nova database slave_connection

Our setup only use one galera node for openstack, due to this issue:
http://osdir.com/ml/openstack-dev/2014-05/msg01269.html

But nova allow to run certain safe read request on other nodes

We can enable this feature by using:

cloud::compute::nova_db_use_slave = true
cloud::loadbalancer::galera_slave = true

The loadbalancer creates a new mysql service on port 3307 that uses only
backup galera nodes (and the main node as backup) to serve nova slave
connection.