cherry pick to track1: disable GCP tests (#2715)
cherry pick to track1: disable GCP tests (#2715)

* tests/ci: disable GCP tests (#2599)

* tests: Comment out GCP Jenkins parameter to reduce confusion (#2716)

aknuds1/tectonic-installer,aknuds1/tectonic-installer,coreos/tectonic-installer,coreos/tectonic-installer,coreos/tectonic-installer,aknuds1/tectonic-installer,aknuds1/tectonic-installer,coreos/tectonic-installer,aknuds1/tectonic-installer,coreos/tectonic-installer