ml2: fix neutron-server restart

Only restart neutron-server on nodes hosting this service.
The plugin class is not linked to neutron-server so it should not depend
on it in the catalog.

Closes-bug #1334906

Change-Id: I6781a8f60c04d7ef18202d378aa096c794e3fa44
(cherry picked from commit cfd0abbab040fe75144f4e7814de530bfbf4c538)
Merge "ml2: fix neutron-server restart" into stable/icehouse