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.

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.

With this commit, bootstrap-vz fully passes GCE's routine automatic validation tests
for the first time! Once this is merged, I'll proceed to build what will hopefully be the
first GCE image officially published to the debian-cloud project built with bootstrap-vz.
We'll allow lots of other internal teams' cases a bit of time to hammer on it first and find
problems our tests might not have caught, just in case.