#60 resolution: added container awareness to prevent "supervise/ok" bloc...

If you run runit cookbook under docker container it will stuck. runit provider will fall into infinitive loop waiting for spuervise/ok (which will never happen for docker). While OHAI is not container aware yet, current PR adds checks if cookbook convergence happen inside container and skips supervise/ok wait.

Should work for different types of containers not only for docker where it was tested
Rather than shelling out to `cat`, use Ruby's `IO.read` to read in the file content. Also check that the file exists first otherwise this will cause an exception. 

Understandably this is tricky to test in post convergence (kitchen verify, eg), but this method should have spec tests too.