Create job to manage Jenkins Global Variables

Parses a configuration file containing variable listings in the form
"KEY=value" one per line and creates the appropriate groovy
script to push these environment variables to Jenkins Global Config.

Configuration file is stored in
ci-management/jenkins-config/global-vars-SILO.sh

(Where silo is production | sandbox)

While JJB provided us a way to store job configuration in a git repo
we have never had a good way to manage global Jenkins configuration.
This patch enables us to manage one aspect, Global Environment
variables in the ci-management repository.

Issue: RELENG-753
Change-Id: I7d134950b25d469686cace2f79fd8c297727166c
Signed-off-by: Thanh Ha <thanh.ha@linuxfoundation.org>