Rebind security groups only when they're updated

Update the security port bindings for a port only when they actually
differ from the stored value.

This will also avoid sending port_update notifications to the agent
even if nothing actually changes in the port configuration.

Closes-Bug: #1255183
Partial blueprint: neutron-tempest-parallel

Change-Id: I00c29dc97c46478433fdf08069a884bb78e5cd0a
Merge "Rebind security groups only when they're updated"