DBaas: Disk size is always 0 for Percona XtraDB cluster

User Impact: User can be confused with zero Disk Size
Steps to reproduce

1. Register Kubernetes cluster with installed PXC operator
2. Create Percona XtraDB cluster with specified Disk size=25
3. Wait until the new DB cluster will have "Active" status

Actual result: Disk Size: 0

Expected result: Disk Size: 25

Proposed solution:

The  storage values do not pass correctly from UI to k8s:

https://github.com/percona-platform/dbaas-controller/blob/main/service/cluster/xtra_db_cluster.go#L121

So during the creation of PXC, it used default value 1G.

And as a result, there is an issue during conversion https://github.com/percona-platform/dbaas-controller/blob/cb78e257cfc95f56b2ef78ca9b298d32f0470001/service/cluster/xtra_db_cluster.go#L72

from 

 "resources": {
                "requests": {
                  "storage": "1G"
                }
              }

and we receive 0.