populate Katello.config.katello_version instead of it being nil

note: every ruby page on the internet that talks about global variables
says to use `$CHILD_STATUS` instead of `$?`, but `$CHILD_STATUS` always
came up as `nil` for me whereas `$?` correctly returned a `Process`
object.
Merge pull request #3546 from komidore64/version-display

populate Katello.config.katello_version instead of it being nil