Fix assumed port has port_security_enabled

Previously if adding a security group to an instance it would fail
if the port did not contain port_security_enabled=True. This patch
also changes the unit tests so that networks by default are not
associated with port_security_enabled=True because this is only true
if a plugin is running the port_security extension.

Fixes bug 1189059

Change-Id: Ieebb7aafba01c18164b86a01a3c289a6957321e4
(cherry picked from commit eddab7ba0e45c47fa8503b29484ce296f46c0d27)