Fix up some docstrings.

The version.py docstrings were a) not PEP compliant and b) somewhat
out of sync with the code.

Change-Id: I87c4134ef80fd9e6fd57b66c58c3c5920ee0b6b6
Return the real class in VersionInfo __repr__.

Change-Id: I52337fd704a0d4b38f9562d88dcdf5952cf7b422
Factor run_cmd out of the base class.

I want to reuse this for factoring out some fixtures, so make it
reusable.

Change-Id: I7035a514adf889a8ebaadff9ae873341804fe099