Create a nova-docker repo

The docker driver has been removed from Nova while CI continues to be
sorted out and stabilized.  We would like to allow development to continue
on this driver as a nova add-on.

Driver removal was here: https://review.openstack.org/#/c/79740/

Change-Id: I560597f1c1d044957dfb640786761ba121941672
Merge "Create a nova-docker repo"