Check for ports in subnet before deleting it from Nuage VSD

There could exist a port in subnet in neutron in which case subnet delete
is not allowed. In the nuage's plugin code, this validation needs to
be handle prior to sending delete subnet request to backend.

Change-Id: Idd20d2f878a6274a012504108d01bd2237dc665a
Closes-bug: #1351522
Merge "Check for ports in subnet before deleting it from Nuage VSD"