Docker Machine: Only Ubuntu is supported for now

Related:
- https://github.com/docker/machine/pull/18#issuecomment-66693839
- https://github.com/ggiamarchi/machine/commit/dfb3f3a5dbb015fbf8dabeac8491af0d23a28329

Signed-off-by: Vincent Giersch <vincent.giersch@ovh.net>