ovs-agent: Ensure integration bridge is created

ovs-agent will fail to launch if integration bridge(br-int by default) is not
existed. To fix this, the agent should check and create the bridge if necessary
during launching.

Cherry-picked from https://review.openstack.org/#/c/98755/

Closes-Bug: 1328076
Change-Id: Iaa6c4d766ca086deb28c8ef7310dbfdcef2c105f
Merge "ovs-agent: Ensure integration bridge is created" into stable/icehouse