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Add keyring parameter in Ceph RBD provisioner

**What this PR does / why we need it**:
userSecretName is a mandatory parameter for the kubernetes.io/rbd storage provisioner. Ceph RBD persistent volumes however don't need the secretRef. If no secret is given, a local keyring file (/etc/ceph/keyring) on the node will be used. This behaviour should also work with the storage provisioner. If no userSecretName parameter is given, no secretRef should be created in the resulting persistent volume.

**Special notes for your reviewer**:

**Release note**:
```release-note
Add "keyring" parameter for Ceph RBD provisioner
```