[JENKINS-23084] support abusolute in file parameters

[JENKINS-23084] (https://issues.jenkins-ci.org/browse/JENKINS-23084)

Allow using absolute paths in file parameters.
Absolute path have already worked correct in the past releases. This change updates help texts and adds tests to verify the behavior.

This also allow file parameters work even without workspaces (which is the case used with [Workspace Cleanup plugin] (https://wiki.jenkins-ci.org/display/JENKINS/Workspace+Cleanup+Plugin), which is reported in [JENKINS-22229] (https://issues.jenkins-ci.org/browse/JENKINS-22229), #64.