Get statistics only for running containers. Fix #1281

[NITEMAN]

Stats only has sense for running container anyway

[sumpfralle]

Thanks for your proposal!

Do you think, that it could make sense to apply the same condition to all_containers in general?

(btw.: maybe a list comprehension could be more readable, than filter?)

[NITEMAN]

I think the condition only has sense in stats, you'll probably want status of all_containers

Regarding list comprehension... I don't use to code python so I just googled, landed on filter, tested and saw it works ;)... Let me see if I can write the same using list comprehension

[NITEMAN]

@sumpfralle converted to a list comprehension and tested.

BTW: It seems that all_containers should already be doing the filtering... but my brand new python skills gime me no idea of what's going wrong (maybe a cache?)