Handle failures on update_dhcp_port

Ensure exceptions due to conflicting
state of network or subnet resources
are dealt with by the dhcp agent.

Closes-bug:  #1253344
Related-bug: #1243726

Change-Id: I4fd51442c034fabc91d5a3f065f4df98f5fad35b
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"
Merge "Handle failures on update_dhcp_port"