This does local backups of the nodepool zk image image data to
/var/log/nodepool on the nodepool-builders. These hosts don't get
offsite backups but we run mutliple redundant servers. This data isn't
critical as we can start from scratch, but may be useful if we don't
want to go through all that trouble.
Change-Id: I7d150df9c0d9566ef2d32167cea535e29822cfa2
a1cf5b3f6f