Added docker_container 'restart' attribute (no, on-failure, always).

Since docker 1.2.0, automatic restart of containers is no longer done with a global daemon option #237 but with explicit [restart policies](http://docs.docker.com/reference/commandline/cli/#restart-policies). This PR adds a new `restart` attribute to the docker_container LWRP.

Attention: for a given container, the `restart` and `init_type` attributes should be incompatible.