Bump docker to 1.12 in bootstrap-servers
Bump docker to 1.12 in bootstrap-servers

I've seen issues with API versioning between docker 1.11 and docker-py
in containers. 1.12 seems fine. We should also pin docker-py version in
kolla-toolbox image.

Change-Id: I07d5f573176b79a3775dcab2748f428c114bd985

dardelean/kolla-ansible,dardelean/kolla-ansible,dardelean/kolla-ansible