beta: puppet rebase script

Script to rebase local checkout of operations/puppet.git on
origin/production branch. The script tries to preserve local changes in
the form of cherry-picks, local-only commits and dirty working copy
files. Also adds a cron job to run the script once an hour. Messages
from the update are logged to /var/log/git-sync-upstream.log and managed
with logrotate.

If this proves to be mostly stable in beta it might be nice to offer it
as an optional class for any labs hosts using role::puppet::self.

Bug: 66683
Change-Id: I8072780a48e27d3d4271a82660417d711b8e2082