Assume VNIC_NORMAL if binding:vnic_type not set

Nova was assuming that every port returned from neutron had a
binding:vnic_type attribute, however this is not always the case.
This fix ensures that in the cases when a port does not have the
attribute nova assumes the default, VNIC_NORMAL. Unit tests added
to prevent the regression of this change have also been addded.

Change-Id: Ia92ffa8658c2e972fa5556e9e47ada8affb57053
Closes-Bug: 1370077
Merge "Assume VNIC_NORMAL if binding:vnic_type not set"