ml2 extension driver: more tests, fix data argument inconsistency

ml2 extension driver: more tests, fix data argument inconsistency

This patch adds more tests for ML2 extension driver.
It also fixes a minor bug which was revealed.

The data argument for process/update method of extension driver
was inconsist. some are given data like
{'resource': {'arg': 'value'...}}. But some are given one like
{'arg': 'value'}.
This inconsistency needs to be fixed so that argument is
{'arg': 'value'}. Given the argument is known to be network,
there is no point to carry outer dictionary.

Partially Implements: blueprint ml2-ovs-portsecurity
Change-Id: I4614c3ba5eff0ace46cc928517e31c14b7b2e448