Fix wrong configuration for `service_plugins`

Change `service_plugins =
neutron.plugins.services.agent_loadbalancer.plugin.LoadBalancerPlugin`
to
`service_plugins =
neutron.services.loadbalancer.plugin.LoadBalancerPlugin`

Fixes bug #1257210

Change-Id: Ida84acc3ba3d167a3a39c38f55b0c9de781f1b56
Merge "Fix wrong configuration for `service_plugins`"