Provide a docker "compute vm"

With Solum we have two choices to run an application
1 - in a VM
2- in a Docker container

for #2, we certainly would prefer using the nova docker driver rather than using directly doker.
This means that we will need at least one compute node installed with the Docker Driver. Doing so implies it is capable of running only docker containers and not vms. OpenStack operators will have multiple choices
1 - Disable Docker and allow only the "VM" choice for Solum (choice #10
1 - have a bare metal compute node dedicated to spawn docker containers
2 - Start an OpenStack VM containing nova-compute services and configured with the Docker driver and use it as a compute node

I think it is solum's responsibility to provide such a vm, in order to allow operators with an openstack installation to be able to run Solum without having to change their cluster configuration