Improve discovery and error messages in create_certificates.sh.
Improve discovery and error messages in create_certificates.sh.

Print error message if openssl/keytool cannot be found.

mp911de/spring-cloud-vault-config,spring-cloud-incubator/spring-cloud-vault-config,mp911de/spring-cloud-vault-config,spring-cloud-incubator/spring-cloud-vault-config,spencergibb/spring-cloud-vault-config,spencergibb/spring-cloud-vault-config