cephrgw-system: source cephrgw-credentials from Vault via VSO

Replace the out-of-band cephrgw-credentials Secret with a VSO-managed one so
the operator's Ceph dashboard credentials come from Vault.

- Add apps/base/cephrgw-system/vaultauth.yaml: VaultAuth (k8s/au/syd1 mount,
  role cephrgw-operator, SA cephrgw-operator, connection vso-system/default).
- Add apps/base/cephrgw-system/vaultstaticsecret.yaml: renders the KV path
  service/cephrgw/dashboard-credentials into the cephrgw-credentials Secret
  (keys copied verbatim, consumed by the Deployment via envFrom).
- Reference both from the base kustomization.

Requires the Vault role/policy from terraform-vault #95 and the KV values to
be seeded (see the operator's docs/ceph-setup.md).

apps/base/cephrgw-system/kustomization.yaml

@@ -9,3 +9,5 @@ resources:
   - https://git.unkin.net/unkin/cephrgw-operator/raw/tag/v0.1.0/config/crd/install.yaml
   - rbac.yaml
   - deployment.yaml
+  - vaultauth.yaml
+  - vaultstaticsecret.yaml