[BUG] KB_POD_FQDN does not include cluster domain

[loomts]

Describe the bug

As the k8s doc said, pod DNS can be the structure of FQDN like hostName+serviceName+namespace+clusterDomain.

And in KB implementation, the service of instanceset is "headless service", pod FQDN should be something like <hostname>.<serviceName>-headless.<namespace>.svc<clusterDomain>.

However, the environment variable KB_POD_FQDN, KB_LEADER_POD_FQDN, KB_SWITCHOVER_CANDIDATE_FQDN and HEADLESS_SVC_FQDN do not include cluster domain right now, maybe need to modify them and change api docs and usage of addons.

Screenshots