change the flex wrapper volume driver for kubernetes to talk directly to
docker instead of reusing the cvmfs library logic. Reasoning is to also
reusing the logic of interpreting volume names when tags or hashes are
used, as in:
* atlas.cern.ch@trunk-previous
* atlas.cern.ch#somehash

It also makes the code a lot simpler.

In addition, add proper support for govendor usage, and document it.