Metadata plugin module - largely based on Parameters module.

I implemented this plugin as I use Jenkins Metadata plugin heavily in all my
projects instead of the default Parameters plugin and think that everyone else
would benefit from this addition to JJB as well.
Current implementation handles dates in epoch format only and does not handle
'node' element - so no metatree creation is possible.

Change-Id: I1e67224df9339f1319a371d049585c8343301f2e
Reviewed-on: https://review.openstack.org/33773
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins