Commit 31fa23d84c9054fb4cfebccef953bf6839698a1d changed the way that
subnet deletion worked. This broke the plugin due to the fact that
port was updated when a subnet is deleted. This results in an
exception for port security validations.
So here we have a dodgy way of finding out that we need to skip this
validation
Change-Id: Idf703cb8d5618799306c6e3b4ab144abb0caa665