Previously if extension was not supported by one of the mech drivers,
but it wasn't filtered out by next mech driver, it was available finally
in the list.
Now, this patch changes that so if extension is disabled by one of the
drivers it isn't available on the list at all.
This will work better e.g. with discoverability of what is available
e.g. when OVN backend is used by Neutron.
Closes-Bug: #1929676
Change-Id: I6a4ff42f47f7ee90365516d37472c09ac87773e5
db2207f32d