Fix external network configurations for Mellanox

The external network type has been changed to local,
as a part of the fix of the following bugs:
bug/1272349
bug/1357298
bug/1357292
bug/1357298
bug/1260051

In This commit 'local' type is added to the ML2 type_drivers,
in case of choosing Mellanox, in order to support external
network changes.

Closes-bug: #1369988

Change-Id: I580a9e87550140a66eaf685dc0c4832c2e135463
Signed-off-by: Aviram Bar-Haim <aviramb@mellanox.com>