`keystone_service` resource

This resource sets up a Docker container with Keystone code. It requires config to be provided by the keystone_config resource (mounted under /etc/keystone).

Basically, the philosophy behind containers in Solar is to have stateless containers with service code and mount stateful resources with config, volumes, etc. to that container. Upgrade of code then would be just about replacing the stateless container with new one and remounting state to that new container.