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.
make-ca-cert.sh already has a mkdir -p.  Were you expecting that make-ca-cert.sh is supposed to run first?  If so, the recently filed #2365 may be relevant.