[cert-management]

Improvements

[USER] allow to set certificate secret reference without setting 'secretName' field (#30, @MartinWeindel)
[USER] validate for duplicate domain names in certificate (#30, @MartinWeindel)
[OPERATOR] allow to specify a quota for certificate requests per day (#30, @MartinWeindel)
[OPERATOR] lease mechanism is activated for all controllers to allow running multiple instances (#30, @MartinWeindel)
[OPERATOR] The commonName field of certificates is now subjected to a 64 characters limit. (#28, @timuthy)
[OPERATOR] Validation for CRDs are added. (#26, @zanetworker)