with I1c9bea2fdeebc4199c4f7d8fca4580a6fb7fed5b nova removed
docker from it's driver tree.
We shouldn't have driver support inside of devstack that's not
part of upstream projects (this has been a line we've been
pretty clear on with Neutron drivers in the past).
Remove docker driver accordingly.
Change-Id: Ib91d415ea1616d99a5c5e7bc3b9015392fda5847