Add create_cgitrepos.py file

The create_cgitrepos.py is used to generate a cgitrepos config file
from projects.yaml for cgit to use.

Change-Id: I54e09634f1cbb7595d7db629ce7e16a883c5f5b6
Reviewed-on: https://review.openstack.org/35856
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