Can't create a vlan transparent network in mixed driver environment

If I use several ml2 drivers, f.e. openvswitch and cisco, one that supports vlan-transparent extension and another that doesn't, then I can't create a vlan-transparent network at all, because of check_vlan_transparency ml2 driver manager behavior.

What we should do instead of requiring that all drivers support the feature is:

1. always allow to create such networks;
2. fail on port binding attempt for a driver that doesn't support the feature; neutron-server will then try the next driver in line, and hopefully will succeed.

There may also be some work on nova scheduler side to make sure that a transparent port lands on an appropriate node, but that's a separate issue to follow up.

Note: once the bug is fixed, we should be able to safely revert https://review.openstack.org/#/c/359919/ that arguably was a mistake, since SR-IOV driver doesn't support the feature, and would require additional configuration on libvirt side, as described in https://serverfault.com/questions/805992/is-vlan-tagging-on-guest-allowed-in-sr-iov-on-kvm

Another candidate to revert check_vlan_transparency from True to False is linuxbridge driver, that I believe also doesn't support the feature (but someone with more knowledge about the driver should check it first.)