[Bug] impossible to deploy NKE cluster with etcd using disk smaller than 120GB

[Golgautier]

Describe the bug
If you deploy a cluster and specify disk size smaller than 120 GB for etcd, it fails. With GUI, we can deploy ETCD with 40 GB disks.

To Reproduce
Deploy karbon cluster with these parameters for ETCD

        etcd:
          num_instances: 3
          cpu: 4
          memory_gb: 8
          disk_gb: 40

It should fail with this error :

fatal: [localhost]: FAILED! => {
    "changed": false,
    "error": "disk_gb cannot be less then 120",
    "invocation": {
        "module_args": {
            "cluster": {
                "uuid": "0005f824-09ff-ea11-671c-ac1f6b3b4ac8"
            },
            "cluster_type": "PROD",
            "cni": {
                "network_provider": "Flannel",
                "node_cidr_mask_size": 24,

Expected behavior
Allow 40 GB disk size on ETCD servers.