fix(fluentd): Use buffer volume mount name from spec instead of default

[cbuto]

Fixes #1368

This PR updates the fluentd buffer metrics sidecar to use the configured buffer volume name instead of the default. Prior to this change, updating the bufferStorageVolume.pvc.source.claimName would result in the buffer metrics sidecar attempting to mount in the wrong volume name because of the hardcoded const value.

tested with:

apiVersion: logging.banzaicloud.io/v1beta1
kind: Logging
metadata:
  name: all-to-file
spec:
  fluentd:
    bufferStorageVolume:
      pvc:
        source:
          claimName: buf
        spec:
          accessModes:
            - ReadWriteOnce
          resources:
            requests:
              storage: 10Gi
          volumeMode: Filesystem
    bufferVolumeMetrics:
      port: 9200
      path: /metrics
      prometheusAnnotations: true
  controlNamespace: default
  fluentbit: {}

[pepov]

@cbuto Nice, but please add a check to see if we are dealing with a pvc or not (not flexible but should still be supported): 54fdce9

[cbuto]

@cbuto Nice, but please add a check to see if we are dealing with a pvc or not (not flexible but should still be supported): 54fdce9

@pepov thanks, good call. I didn't know that was an option 😄

[OverOrion]

LGTM, thank you for contributing!