Document the OVS restart for the Ryu agent

When installing Quantum Ryu agent, the OVS service might not be properly
started due to a missing kernel module (compiled using DKMS). Make sure
that the OVS service is (re)started before restarting the Ryu agent
(completes commit 9489e66).

Cherry-picked from https://review.openstack.org/34230

Change-Id: I659415bb7514cf5ac3619141e715351692c80854
Merge "Document the OVS restart for the Ryu agent" into stable/grizzly