Sync dhcp_agent.ini with the codes

Add an option item into dhcp_agent.ini to sync with the codes.
* Add ovs_integration_bridge in /etc/dhcp_agent.ini
* Add corresponding unit test

Change-Id: Ia78e7ca6c1fcbeca3e962f870a393e44183d719c
closes-Bug: #1256197
Merge "Sync dhcp_agent.ini with the codes"