kube-cross: Zero out substitution values in GCB config
kube-cross: Zero out substitution values in GCB config

Here we explicitly provide fake values for each of the variables in the
kube-cross GCB config to prevent accidental builds that circumvent the
Makefile or variants.yaml configs.

Signed-off-by: Stephen Augustus <f96984fef9637085f0559f1484dfd4e99545cd6e@vmware.com>

kubernetes/release,kubernetes/release