expose column names by condor version

this adds a json file into condor that contains the columns used for the tracking data
by the (major.minor) condor version. This means that if a column moves in a later version
queries can still run. condor already exposes it's version in a column, so this closes
that set of information - otherwise any code that worked with only one condor version
would have to be manually rewritten.

Also, I have only exposed the major and minor versions. First I did M.m.p but then
I started writing this pull request and I realized that the first thing you'd do was create
and publish a new version and that the version in the file would be wrong. Any additional
columns would just be minor versions, and any moved columns would be major versions so
this should be enough.

Of course, if you decide to merge this, you are implicitly committed to maintaining this file
and adding the columns for new versions ;)