Add option to define secret name for KeystoneUser

This can be useful for e.g. CephObjectStore:

---
apiVersion: yaook.cloud/v1
kind: KeystoneUser
metadata:
  name: swift
spec:
  keystoneRef:
    kind: KeystoneDeployment
    name: keystone
  secretName: swift-admin
---
apiVersion: ceph.rook.io/v1
kind: CephObjectStore
metadata:
  name: swift
spec:
  auth:
    keystone:
      serviceUserSecretName: swift-admin
...