Document the OVS service restart

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

Change-Id: Ic049b1698c2176b2adfa5f32ca6253b6484d966a
Merge "Document the OVS service restart"