ovs-docker: Enhance documentation to handle pre-programmed interfaces.

Currently, Controllers and CMSes that integrate with Open vSwitch
after following the IntegrationGuide.md, expect to start VMs after
the underlying network infrastructure is ready. I have been asked
a few times on how to handle the same with containers. This commit
provides documentation on one way to achieve it.

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>