charts/timescaledb-single: allow to use existing pvc for persistent volumes

[joschiwald]

What this PR does / why we need it

This PR will add the option to use existing persisitent volume claims for the persistent volumes. This is useful in environments where your admin already created the persistent volume for you and you are not allowed to use dynamic volume provisioning.

Which issue this PR fixes

• 624

Special notes for your reviewer

Tested with these values:

values.yaml

```yaml persistentVolumes: data: enabled: true existingClaim: data-pvc wal: enabled: true tablespaces: example1: existingClaim: example1-pvc subPath: "tse1" example2: annotations: {} accessModes: - ReadWriteOnce existingClaim: "" size: 5Gi subPath: "" storageClass: gp2 ```

Checklist

• [x] Chart Version bumped
• [x] CLA signed

[YarekTyshchenko]

This would be a useful feature, what is required to push this through?