pkg fixes cleaned up from #1240

I did a review of the fixes I did during my debuging session, and pushed to 1240. I cleaned up to put them one by commit for easier review.

Several fixes are missing compared to #1240:

- check the revision of distribute. I suspect that python-distribute <  0.6.30 gives install issue, I'm not sure yet exactly how to resolve it. This package is obsolete anyway, one of the possible answer is to just check import it, and create error message to request to cleanly remove it.

- docker install tests. I did not port this patch as it did not really help to track any issue while virtualenv did. It does not run on travis (yet?). I may introduce it again when we upgrade metabuildbot infra or if travis supports it.


note that this leads to a travis test suite that is running in 30min... 

https://travis-ci.org/buildbot/buildbot/builds/38261026

let me know if it is not acceptable