admission-webhook.cstor.openebs.io validation of compression arguments in CStorPoolCluster

[mazocode]

The admission webhook expects values that are invalid for the ZFS compression setting.

Example:
Create a new CStorPoolCluster

apiVersion: cstor.openebs.io/v1
kind: CStorPoolCluster
metadata:
  name: cstor-hdd-x1-gli1-pool
  namespace: openebs
spec:
  pools:
  - dataRaidGroups:
    - blockDevices:
      - blockDeviceName: blockdevice-c89ddb7adce61f8d21b6e27a488b1940
    nodeSelector:
      kubernetes.io/hostname: iuck8s1.core.idnt.net
    poolConfig:
      dataRaidGroupType: stripe
      compression: lz4 # <- lz4 ./ lz 

Result
Failed CStorPoolInstance:
Failed to create pool due to 'Failed to create pool {cstor-6c7f1f4b-9466-4239-8743-cf0cea515df9} : Failed to create pool.. cannot create 'cstor-6c7f1f4b-9466-4239-8743-cf0cea515df9': 'compression' must be one of 'on | off | lzjb | gzip | gzip-[1-9] | zle | lz4'

lz4 or on gets rejected by the webhook:
Error from server (BadRequest): error when creating "STDIN": admission webhook "admission-webhook.cstor.openebs.io" denied the request: invalid cspc specification: invalid pool spec: unsupported compression 'lz4'
Error from server (BadRequest): error when creating "STDIN": admission webhook "admission-webhook.cstor.openebs.io" denied the request: invalid cspc specification: invalid pool spec: unsupported compression 'on' specified

[prateekpandey14]

closing .. fixed in #241