Implement Mellanox ML2 MechanismDriver

This commit adds support for currently provided Mellanox Plugin
embedded switch functionality as part of the VPI (Ethernet/InfiniBand)
HCA as an ML2 MechanismDriver.
MechanismDriver adds support for VNIC_DIRECT and VNIC_MACVTAP vnic types.
MechanismDriver provides configurable default vif_type for neutron port created
with default VNIC_NORMAL vnic type till nova api support for vnic_type is available.

Implements blueprint mlnx-ml2-support

Change-Id: I16ad318f095b7af879e1b99dcc7f5f9e92facd2b
Merge "Implement Mellanox ML2 MechanismDriver"