Description of problem:
When we create a migration plan, and we click on "back" to edit the migration plan destination name, the default storage class is not shown any more.

Version-Release number of selected component (if applicable):
MTC 1.5.0
TARGET CLUSTER: AWS OCP 4.8 (gp2)
SOURCE CLUSTER: AWS OCP 3.11 (gp2)
REPLICATION REPOSITORY: AWS S3

How reproducible:
Always

Steps to Reproduce:
1. Deploy a sample django application
 $ oc new-project bz-test
 $ oc new-app django-psql-persistent

2. Click on "Add migration plan" in the migration plans screen
3. Fill in the form: name, source cluster, destination cluster and replication repository
4. Check the bz-test namespace (do NOT map the namespace to another name) and click "Next"

Once the PVCs are discovered we have this information in the migplan
  namespaces:
  - bz-test
  persistentVolumes:
  - capacity: 1Gi
    name: pvc-69a1dd43-c21c-11eb-a9dd-0eeaed03c931
    proposedCapacity: 1Gi
    pvc:
      accessModes:
      - ReadWriteOnce
      hasReference: true
      name: postgresql
      namespace: bz-test
    selection:
      action: copy
      copyMethod: filesystem
      storageClass: gp2


5. Click on "Back", in order to edit the namespace mapping
7. Click on the pencil icon, and map the namespace to the new name bz-test-map. Save the change clicking on the "V" symbol and click on "Next"

Once the PVCs have been discovered again, we have this information in the migplan

  namespaces:
  - bz-test:bz-test-map
  persistentVolumes:
  - capacity: 1Gi
    name: pvc-69a1dd43-c21c-11eb-a9dd-0eeaed03c931
    proposedCapacity: 1Gi
    pvc:
      accessModes:
      - ReadWriteOnce
      hasReference: true
      name: postgresql
      namespace: bz-test
    selection:
      action: copy
      copyMethod: filesystem
      storageClass: ""
      verify: false
    storageClass: gp2

8. Click on "Next"

Actual results:
The "Copy options" screen does not show the default storage class. The value shown for the storage class is "None", instead of the "gp2" value that should be displayed.

The information that we get in the migplan at this point is this:

  namespaces:
  - bz-test:bz-test-map
  persistentVolumes:
  - capacity: 1Gi
    name: pvc-69a1dd43-c21c-11eb-a9dd-0eeaed03c931
    proposedCapacity: 1Gi
    pvc:
      accessModes:
      - ReadWriteOnce
      hasReference: true
      name: postgresql
      namespace: bz-test
    selection:
      action: copy
      copyMethod: filesystem
    storageClass: gp2


We can notice that the selection.storageclass value is missing from the yaml information.

Expected results:
The default storage class should be displayed in the "Copy options" screen. And the data in the migplan yaml information should be the right one, including the default storage class. 

Additional info: