Less Exception with Docker check

Aims at fixing #980 and #981 

* Warning if no containers instead of exception
* Log if fail to open a cgroup file but keep going. If we fail to open them all, then raise the Docker 0.9/0.10 bug Exception.
Is there a case where we could have multiple instances of the docker check running (i don't think so)? If so this will break because the self.skipped_cgroup will be global to all instances.

A better practice would be to make self.skipped_cgroup a defaultdict whose keys are unique per check instance.