Use Infinispan CR spec data instead of status to fetch credentials Secret

See: https://github.com/infinispan/infinispan-operator/issues/754#issuecomment-765387941

Instead of fetching the status.security.endpointSecretName we need to use the following method from Infinispan types: https://github.com/infinispan/infinispan-operator/blob/a68e32a48150212ade0599dd9111237592387795/pkg/apis/infinispan/v1/types_util.go#L172-L178

This read should be done only if spec.security.endpointSecretName is empty.