Allow to set rpc_workers in neutron barclamp

[tpatzig]

• currently we have rpc_workers = 0, which means it is handled in one single process
• the RPC worker run on the neutron-server side, so this setting is only relevant for control-nodes
• the RPC workers process the messages from rabbit and thus are responsible for the neutron-server - neutron-agent communication
• we want to increase the workers, to increase e.g. l3 router initialization and faster ovs flow retrieval

[matelakat]

needs backport