Add L2 Agent side handling for non consistent security_group settings

Add setting of the firewall_driver to NoopDriver when firewall_driver is None and
add warning if driver combination is not valid.
Modify is_valid_driver_combination to verify default settings: enable_security_group (True) and firewall_driver (None).

Change-Id: I841f9cf96ac6ee2ad17a4e8908d6c8a96f368cca
Closes-Bug: #1296957
(cherry picked from commit 5c6ff449bbd7386f0f3e41efc524024434f325df)
Merge "Add L2 Agent side handling for non consistent security_group settings" into milestone-proposed