Fix typo in AdjustExpandRootScript

This commit fixes a typo in common.tasks.initd.AdjustExpandRootScript (a
missing double-quote in the search regex) that rendered it ineffective.
Enable auto-rootdisk growth on gce-backports.

This commit adds the cloud-initramfs-growroot package to the
installation list for GCE images with backports enabled, and updates
the gce tasklist to add the /etc/init.d/expand-root script
(with provider-appropriate touch-ups) to the image.
Merge pull request #150 from timbot/debian-backports-growroot

Enable auto rootdisk growth on gce-backports