Add Release plugin

Add support for a 'release' wrapper section which enables the
Release Plugin
(https://wiki.jenkins-ci.org/display/JENKINS/Release+Plugin).

Change-Id: I48e9cdd3f32c4f7e3da843b6adb58767b357b2e7
Reviewed-on: https://review.openstack.org/31650
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins