cluster_config argument missing in prepare()
cluster_config argument missing in prepare()

Due to a bad merge, the prepare() call in postgresql does
not have cluster_config argument. Added that to make sure
postgres is in line with other data stores.

Change-Id: I1c3e5032447124dae47ae23ebf820fdcf8080588
Closes-Bug: #1380745
mmasaki/trove,cp16net/trove,zhujzhuo/openstack-trove,fabian4/trove,fabian4/trove,zhujzhuo/openstack-trove,hplustree/trove,openstack/trove,zhangg/trove,openstack/trove,cp16net/trove,hplustree/trove,redhat-openstack/trove,zhangg/trove,zhujzhuo/openstack-trove,mmasaki/trove,mmasaki/trove,fabian4/trove,redhat-openstack/trove,cp16net/trove,redhat-openstack/trove