ML2 plugin should not delete ports on subnet deletion

On subnet deletion ports are deleted asynchronously by dhcp agent
so plugin doesn't need to delete them itself.
Instead Ml2Plugin.delete_subnet() method should call update_port()
for each allocation to remove the IP from the port and call the MechanismDrivers.
The patch also adds subnets test suite from test_db_plugin to test_ml2_plugin.

Closes-Bug: #1246737
Change-Id: I7cf0461e9a3cfec4921e2de41fb1ab3fc119fddc
Merge "ML2 plugin should not delete ports on subnet deletion"