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.
```python
try:
    foo
except:
    bar
finally:
    foobar
```
is not compatible with older versions of python. We should try to keep this check compatible with those versions by using the double try magic syntax until we actually release the self contained agent.