multi volumeClaimTemplate support on elasticsearch chart

[batrako]

With elasticsearch , in some cases , we need some extra volumes with your own properties to store snapshots or backups (for example). Current version , only supports one VolumeClaimTemplate. Could be very useful for us, support for multi volumes on elasticsearch group nodes.
Thanks

[Crazybus]

The repository used for taking snapshots needs to be a shared file system.

In order to register the shared file system repository it is necessary to mount the same shared filesystem to the same location on all master and data nodes.

Adding the ability to allow multiple volumeClaimTemplates wouldn't work for a snapshot repository. As it would create a separate persistent volume claim for each node.

Instead it sounds like we need to add the ability to mount extra volumes. This would only work for storage that supports ReadWriteMany. Another alternative is to use one of the repository plugins if one of those backends is available to you.

This is what would need to be added to make this work:

1. Ability to define extra persistentVolumeClaims.
2. Ability to mount persistent volume claims as volumes.

The configuration would look something like this:

persistentVolumesClaims:
  claimName:
    accessModes:
      - ReadWriteMany
    volumeMode: Filesystem
    resources:
      requests:
        storage: 8Gi
    storageClassName: slow
    selector:
      matchLabels:
        release: "stable"

persistentVolumeMounts:
  claimName: /usr/share/elasticsearch/snapshots

[Crazybus]

Support for mounting extra volumes and volume mounts was added in #70.

Can you try it out and see if it works for you? If not feel free to reopen this issue!