Exit gce kube-up.sh early if openssl is LibreSSL

macOS has an openssl binary, but it's actually LibreSSL, which
doesn't play well with the easyrsa script that cluster/gce/util.sh
uses to generate certs

Instead of waiting until we generate certs to discover easyrsa doesn't
work, consider openssl a prereq for gce, and include a check for the
version string starting with OpenSSL

Also, mirror kube-up.sh's "... calling" output in kube-down.sh

cluster/kube-down.sh

@@ -30,8 +30,11 @@ source "${KUBE_ROOT}/cluster/kube-util.sh"
 
 echo "Bringing down cluster using provider: $KUBERNETES_PROVIDER"
 
+echo "... calling verify-prereqs" >&2
 verify-prereqs
+echo "... calling verify-kube-binaries" >&2
 verify-kube-binaries
+echo "... calling kube-down" >&2
 kube-down
 
 echo "Done"