health check for standby returns a failure status

[raoofm]

health check for standby returns a failure status code, causing the second vault instance in HA to be marked as failed, which in turn causes deployment to fail.

https://github.com/coreos/vault-operator/blob/master/pkg/util/k8sutil/vault.go#L189 and https://github.com/coreos/vault-operator/blob/master/pkg/util/k8sutil/vault.go#L201