Add 'restart_policy' to docker module

Since version 1.2.0/ApiVersion 1.14 docker supports the flag
`restart_policy`. docker.py supports that flag since version 0.5.0.

This adds the new module parameter, to take advantage of the progress
on dockers side.

I'm not overly happy with the argument_spec for this, since it should be
a choice, but it also needs the numbered parameter for on-failure (see:
https://github.com/docker/docker-py/blob/master/docs/api.md#start ).

Docker on the other hand seems to happily ignore wrong values for this,
so I cannot see that wrong values for the parameter are going to break
anything.