Optimize stable-ubuntu14.04 Dockerfile

Hi,
as far as I know, best practise for docker seems to avoid running apt-get upgrade.
The base image (in this case ubuntu:14.04) is responsible for upgrades.

Second, having several run commands for apt creates unnecessary layers in the final docker image, it would be better to do something like
   RUN apt-get -y update &&  apt-get install [...]

One last thing I want to discuss:
Installing ansible with pip really pulls a lot of dependencies. 
The size of the final image decreases from 413.4 MB to 292.6 MB with this pull request. (The base image ubuntu:14.04 is 197.8 MB).
And at least for the stable version I prefer using the repository ppa:ansible/ansible.

Best regards,
Martin Schmidt