Remove default value from Port binding:vnic_type parameter

Remove the default value from this parameter so that this
plugin can be used with Neutron services that do not support
the binding extension.

Change-Id: Idb0cae57217905f5f191ed9fe24eabf226b2e7b1
Closes-Bug: 1405266
Merge "Remove default value from Port binding:vnic_type parameter"