LOW-359: implement deviceserver[].volume.existingClaimName

When `existingClaimName` is specified, no PVC is created when the chart is
deployed. The device server pod mounts the named PVC at `volume.mountPath`, and
all other keys under `volume` are ignored. The PVC is assumed to already exist
or to be created elsewhere in the chart.

charts/ska-tango-util/templates/_deviceserver-pvc.yaml

@@ -16,7 +16,7 @@ Parameters are:
 {{ define "ska-tango-util.deviceserver-pvc.tpl" }}
 
 ---
-{{- if .volume }}
+{{- if and .volume (not .volume.existingClaimName) }}
 
 {{ $volume_name := .volume.name }}
 {{ $volume_path := .volume.mountPath }}

charts/ska-tango-util/templates/_multidevice-svc.yaml

@@ -132,7 +132,7 @@ spec:
         {{ if hasKey $deviceserver "volume" }}
         - name: volume-data
           persistentVolumeClaim:
-            claimName: {{ $deviceserver.volume.name | toString }}-pvc-{{ $chart.Release.Name }}
+            claimName: {{ $deviceserver.volume.existingClaimName | default (printf "%s-pvc-%s" $deviceserver.volume.name $chart.Release.Name) }}
         {{ end }}
       serviceAccountName: {{ $deviceserver_name | toString }}-sa-{{ $chart.Release.Name }}
       initContainers: