add arguments to the jenkins upgrade job

The upgrade job (http://ci.aegirproject.org/job/aegir%206.x-1.0-rc3%20to%206.x-1.x%20upgr...) right now upgrades always from 1.0-rc3 to whatever is the latest version on 1.x. It would be nice to have those versions be arguments so that you could test multiple combinations: 1.0 to 1.4, 1.0 to 1.x, etc. This could be then part of the release process - make sure we can upgrade from previous versions!