Add version option to docker_daemon plugin

As requested on #147, there is now an option to specify the Docker
version to be installed when using the `docker_daemon` plugin. The
version string is validated against a pattern extracted from the
Docker's CHANGELOG. If the version is not present, it will just download
the latest available.

The download method was also changed from `urllib` to `wget`, so we can
see its progress if needed.

This closes #147.