Find projects by their relative names in promotion parameter

https://wiki.jenkins-ci.org/display/JENKINS/Hierarchical+projects+support

Using `Jenkins.getInstance().getItem(...)` only returns `TopLevelItem`, which
means that it was previously impossible to use the `Promoted Build Parameter`
with folders. To make that work better, use relative names.

This is an alternative to pull request #47.

(Originally linked to JENKINS-25011 but I think this is addressing an unrelated problem.)
I can remove it from this change, but it would be good to clean that up a bit have more consistency.