Use relative_require in artifactory functions

This doesn't seem to have been neccessary for all users and probably
depends on how puppetserver has ruby-load-path configured.
See https://tickets.puppetlabs.com/browse/SERVER-973?focusedCommentId=224547&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-224547

Requiring the library using an absolute path should help
https://tickets.puppetlabs.com/browse/SERVER-973?focusedCommentId=352751&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-352751

Note, any changes to util.rb will still result in horrible environment
isolation issues. A full fix would involve implementing
@trevor-vaughan's complete metaprogramming solution documented here.
https://www.onyxpoint.com/fixing-the-client-side-of-multi-tenancy-in-the-puppet-server

util.rb is somewhat more complicated than the example demonstrated, so
I've left that well alone for now.

Fixes #320