Reinstate hostname hook for GCE

This DHCP exit hook to shorten the system hostname on GCE was previously
installed by build-debian-cloud and bootstrap-vz, but seems to have been
inadvertently removed in commit c81045cc6e0f6a2c90002ab50219a15c337630f1
as part of a broad cross-cloud cleanup. Again, this was caught by our
validation tests, and we might be done with the fixes at this point.

In this commit, I'm reinstating the hook with a name change and an
explanatory comment, to reduce the risk of this vanishing accidentally
in the future.

Change-Id: I4e7268f8b9ab3b2a7fc8b510898c6fbdd685aa53