Make service_plugins configurable

So far, the service_plugins configuration setting of neutron server has
been used exclusivly for the loadbalancer plugin.  However, in different
usage contexts it can be used for more.  Therefore, this change makes
the service_plugins setting configurable.

This change deprecates node["openstack"]["network"]["neutron_loadbalancer"].
To use the loadbalancer as a service plugin, please add

  neutron.plugins.services.agent_loadbalancer.plugin.LoadBalancerPlugin

to node['openstack']['network']['service_plugins']

Change-Id: I6f1cf72f7bbb9625dbace778189f938841bfbe2b
Merge "Make service_plugins configurable"