Add port-security extension API test cases

Add port-security extension API test cases

Netron ml2 plugin introduces a new extension port-security from Kilo cycle,
this patch add the API test cases for it. It verifies the default value of the
attribute, for network and port. And It also verifies the confict between
setting port-security and sec-group/address-pairs.

Change-Id: Ie0ec090e8fdce7dbdbce14ef47f38e8e57f262d4
Partially Implements: blueprint ml2-ovs-portsecurity
Depends-On: I3035317c83d22804855517434bd8578719ce0436