Fix static number of volumes - fixes #11

[hpdobrica]

Hello there, in order to enable running multiple replicas, each with their own storage, i have:

• removed predefined persistent volume claims
• added volumeClaimTemplates to the statefulset
• ensured memgraph fullname is used both when creating and connecting the volumes

note that this is likely a breaking change which will require reinstall of the chart and migration of data (old single volume was named memgraph-lib-storage, new one would be memgraph-lib-storage-memgraph-$POD_NUMBER , as we are editing the volumeClaimTemplates field:

Error: UPGRADE FAILED: cannot patch "memgraph" with kind StatefulSet: StatefulSet.apps "memgraph" is invalid: spec: Forbidden: updates to statefulset spec for fields other than 'replicas', 'template', 'updateStrategy', 'persistentVolumeClaimRetentionPolicy' and 'minReadySeconds' are forbidden

i have also added the option to define a desired storage class to be used when creating the volumes.

this pr fixes #11

[katarinasupe]

Thank you, @hpdobrica, for your contribution! You're improving the Memgraph experience for our whole community 😄 We will review this PR as soon as possible and get back to you.