Optionally the docker group to the mistral user
Optionally the docker group to the mistral user

This allows the workflow driven container prepare to make local docker
calls, which is required for pull/push calls to work when uploading
images to the local registry.

This is only required for the undercloud mistral, so is only
enabled by setting $docker_group = true.

Change-Id: Ifab466893beb5bfaa0314b50d14740f2b04771a8
Blueprint: #container-prepare-workflow

openstack/puppet-tripleo,openstack/puppet-tripleo,openstack/puppet-tripleo