Perform a joined query for ports and security group associations

bug 1174111

Instead of loading security group associations with a separate query
each time a port is loaded from the database, perform the load operation
with a join using a joined sqlalchemy relationship.
Also, this patch removes the need for invoking the mixin method
'extend_port_dict_security_group' from the plugin code.

Change-Id: I40b22281d45ff4f4bf8149211883799a9051c1a0