* add json:omitempty option to ClusterDomain
* Add default value for ClusterDomain
Unfortunately, omitempty in operator configuration CRD doesn't mean that
defauls from operator config object will be picked up automatically.
Make sure that ClusterDomain default is specified, so that even when
someone will set cluster_domain = "", it will be overwritted with a
default value.
Co-authored-by: mlu42 <mlu42pro@gmail.com>