providers/gce: add InstallInitScript task
Split GCE manifest into regular & backports

Some people want the security team-supported 3.2 kernel, while others
want the better performance of the backports kernel. Both flavors are
currently provided in the Google Compute Engine debian-cloud project.

Also add OpenSSH to the backports flavor, for parity with the current
GCE backports image based on build-debian-cloud. This backport is
included for performance reasons, in line with the purpose of that image
flavor, and as discussed on the debian-cloud list.
Merge pull request #92 from jkaplowitz/gce-manifest-split

Split GCE manifest into regular & backports
Merge pull request #88 from proppy/add-gce-init

add InstallInitScript task to gce provider
bootstrapvz/tasks/bootstrap: add variant option

Change-Id: I757dd4f5e8daa2c4de6f684a152945e407106a4e