zuul: pin GitPython <2.1.2

See: gitpython-developers/GitPython#605

This change also fixes the nodepool health check where the last
image includes the jenkins repository which trigger an unexpected
update of jenkins when slave image is prepared

Change-Id: I60c919cdd987a9f3b8556b8e5f92f0f169cc392c

Author: TristanCacqueray

Diff for: files/nodepool_demo.sh

@@ -1,7 +1,7 @@
 #!/bin/bash
 # This script re-use a software-factory image as a nodepool slave
 
-sudo edeploy activate-pkgmngr
+sudo rm /etc/yum.repos.d/jenkins.repo
 sudo userdel jenkins
 
 # Set tests domain name to the nodepool client ip