Changes for Mellanox ML2 configurations

Added changes:
  1. Add ml2 eswitch section configurations in deployment
     serializers.
  2. Change mechanism drivers in generate_l2 of nailgun neutron.py.

partially implements: blueprint mellanox-features-support

Change-Id: Ia9ab697edd1a364a606bb1a494531fe0ec5f1c1c
Signed-off-by: Aviram Bar-Haim <aviramb@mellanox.com>
Merge "Changes for Mellanox ML2 configurations"