bug fix restart quantum-plugin-openvswitch-agent

In the cookbook-openstack-network/recipes/common.rb, for "openvswitch",
it restarts the service quantum-plugin-openvswitch-agent whether or not
that service is running.  The service would only be running if the openvswitch
recipe was also included.

Fixes: Bug #1210400
Implements: make the restart quantum-plugin-openvswitch-agent conditional

Change-Id: I8801450ac88547e5961bb37f0d1b7c002671c9e8
Merge "bug fix restart quantum-plugin-openvswitch-agent"