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
Imported Translations from Transifex

Change-Id: Icea5a49b16be31eeef34078ef216a4818c253819
Merge "Imported Translations from Transifex"
Merge "ML2 plugin should not delete ports on subnet deletion"
Merge branch 'master' of https://github.com/openstack/neutron into odl_ml2