Option to use a specific repository URL and branch of jenkins_scripts

I added the option for the chroot scripts to use a specific URL and branch for the jenkins_scripts repository. Before, it was hardcoded to use the latest version of the main fork at github.

In order to specify a different URL, simply set the JENKINS_SCRIPTS_REPOSITORY_URL environment variable before running the run_chroot_* scripts.
You can further set a JENKINS_SCRIPTS_REPOSITORY_BRANCH variable if you want to specify a particular branch of the repository.