GCE startup: Regional release .tar.gz support

* In kube-up.sh, create a staging bucket with a location nearest the
zone being created. If new variable RELEASE_REGION_FALLBACK is set
(default false), create multiple buckets and stage to fallback
URLs. (In open source, this path is primarily for testing.)

* In configure-vm.sh, split the URL env variables by comma (if any
extra are present) and retry on the fallback URLs. Also factor the
hash checking into this path rather than outside, since a corrupt
release in a particular geo can be retried in a different geo.

* Remove the local already-staged .tar.gz checks. They've caused
several issues along the way, and with this code path become virtually
unmaintainable. (I could add a sentinel for each bucket it's possibly
staged to, but ew.)

cluster/gce/config-default.sh

@@ -18,6 +18,7 @@
 # gcloud multiplexing for shared GCE/GKE tests.
 GCLOUD=gcloud
 ZONE=${KUBE_GCE_ZONE:-us-central1-b}
+RELEASE_REGION_FALLBACK=${RELEASE_REGION_FALLBACK:-false}
 MASTER_SIZE=${MASTER_SIZE:-n1-standard-2}
 NODE_SIZE=${NODE_SIZE:-n1-standard-2}
 NUM_NODES=${NUM_NODES:-3}