Allow bootstrap re-apply for Fedora CoreOS GCP

* Problem: Fedora CoreOS images are manually uploaded to GCP. When a
cluster is created with a stale image, Zincati immediately checks
for the latest stable image, fetches, and reboots. In practice,
this can unfortunately occur exactly during the initial cluster
bootstrap phase.

* Recommended: Upload the latest Fedora CoreOS image regularly
* Mitigation: Allow a failed bootstrap.service run (which won't touch
the done ConditionalPathExists) to be re-run by running `terraforma apply`
again. Add a known issue to CHANGES
* Update docs to show the current Fedora CoreOS stable version to
reduce likelihood users see this issue

 Longer term ideas:

* Ideal: Fedora CoreOS publishes a stable channel. Instances will always
boot with the latest image in a channel. The problem disappears since
it works the same way AWS does
* Timer: Consider some timer-based approach to have zincati delay any
system reboots for the first ~30 min of a machine's life. Possibly just
configured on the controller node https://github.com/coreos/zincati/pull/251
* External coordination: For Container Linux, locksmith filled a similar
role and was disabled to allow CLUO to coordinate reboots. By running
atop Kubernetes, it was not possible for the reboot to occur before
cluster bootstrap
* Rely on https://github.com/coreos/zincati/issues/115 to delay the
reboot since bootstrap involves an SSH session
* Use path-based activation of zincati on controllers and set that
path at the end of the bootstrap process

Rel: https://github.com/coreos/fedora-coreos-tracker/issues/239

CHANGES.md

@@ -33,6 +33,10 @@ Notable changes between versions.
 
 * Update default `os_stream` from testing to stable
 
+#### Google Cloud
+
+* Known: Use of stale Fedora CoreOS image may require terraform re-apply during bootstrap ([#687](https://github.com/poseidon/typhoon/pull/687))
+
 #### DigitalOcean
 
 * Rename `image` variable to `os_image` for consistency ([#677](https://github.com/poseidon/typhoon/pull/677)) (action required)