Add support to request vnic type on port

Add support to request vnic type on port

This patch adds support for requested vnic_type to be plugged to neutron port to ML2 plugin.
This patch contains:
1. New attribute 'binding:vnic_type' added to port binding extension.
   Possible values are 'direct', 'macvtap' and 'normal'.
   'binding:vnic_type' is allowed to be defined on port creation or changed
   on port update by admin or tenant user.
   'binding:vnic_type' can be also skipped in port defintion
2. Management of vnic_type by ML2 plugin, assuming default
vnic_type=normal
3. Add 'vnic_type' to ml2_port_bindings DB table
4. Add supported vnic_types for MechanismDrivers that are capable to bind
port.
5. Add DB migration script for ml2_vnic_type.

DocImpact: Need to update portbindings API docs and include in SR-IOV user docs

Change-Id: Ic88708fa9ece742f807c1d09bb49e499f99bd092
Implements: blueprint ml2-request-vnic-type