Nodepool should clone the config repo from git.o.o

* .../nodepool/scripts/cache_git_repos.py: Since the projects list
is not being retrieved as part of a git repository, use HTTPS
protocol rather than HTTP so it will be consistent with the way we
retrieve the bootstrap script in prepare_node.sh.

* .../nodepool/scripts/prepare_node.sh: Switch the config clone
operation from the Gerrit server to the Git server farm, since the
former does not support git protocol. Also make it a shallow clone
since we just need the files.

Change-Id: I1dc0697bb3f9832d180b6b1ebffaa563bd886281