Cisco DFA ML2 Mechanism Driver

Part 1:
This commit contains changes to support ML2 mechanism driver for Cisco
DFA. For more details please see the blueprint which has more description and
link to document with requirements.

Part 2: Changes in OVS neutron agent.
(http://review.openstack.org/110065)

Part 3: DFA extension driver.
(http://review.openstack.org/111761)

Part 4: DFA config profile service plugin.
(http://review.openstack.org/111863)

Change-Id: Ib53b6705948e1ed75059b85d8809562d9bb63f65
Partially Implements: blueprint ml2-mechanism-driver-for-cisco-dfa
Support for extensions in ML2

The current ML2 plugin supports only extensions defined in the plugin and it
does not support any extensions in the mechanism drivers. The changes in this
commit allows mechanism drivers to define new extensions.

Change-Id: I28da19fabf6de2e9f0d687f875aaaa24c8bbc4f0
Implements: blueprint extensions-in-ml2
Merge "Cisco DFA ML2 Mechanism Driver"
Merge "Support for extensions in ML2"