make-cert.sh: Create directory before writing cert

Adds a "mkdir -p" to the make-cert.sh.  This fixes an issue where the
script could fail if /srv/kubernetes did not exist previously.  Fixes for https://github.com/GoogleCloudPlatform/kubernetes/issues/2363.
Should we already create dir in cluster/saltbase/salt/generate-cert/make-ca-cert.sh?