Make glideins advertise their version

[vbrik]

It looks like we have a bunch of out-of-date glideins, but it's hard to say which sites/clients are up-to-date and which aren't.

It would be nice if glideins would advertise their version.

I don't think we have a well-defined concept of version. Perhaps use git commit hash?

[dsschult]

We could use the commit hash. Or we could actually make releases and use them for production.