Support tmpfs for /cache dir on vmselect

[matej-bellus-w]

Currently when I tried to set tmpfs like this via values.yml

  extraVolumes:
    - name: cache-tmpfs
      emptyDir:
        medium: "Memory"
        sizeLimit: 50Mi

  extraVolumeMounts:
    - name: cache-tmpfs
      mountPath: /cache

  cacheMountPath: /cache

It says: spec.template.spec.containers[0].volumeMounts[1].mountPath: Invalid value: "/cache": must be unique

This is the line.