Restore linuxbridge-agent compatibility

This patch partially reverts commit
15130cd5fd1688b8984d78136b97bb8de7c32b64.

This commit was desupporting ovs and linuxbridge plugins.
But the ML2 plugin can be deployed with the linuxbridge agent.

The current patch restores the linuxbridge agent configuration
file.

Closes-Bug: #1393429

Change-Id: If824185b22e22a1de6498f1f0f2742a279235675
Merge "Restore linuxbridge-agent compatibility"