Description of problem: On a RHOS 16.2 deployment, following cinder tempest plugin tests fail while validating schema:

cinder_tempest_plugin.api.volume.admin.test_volume_backup.VolumesBackupsTest.test_backup_crossproject_admin_negative
 cinder_tempest_plugin.api.volume.admin.test_volume_backup.VolumesBackupsTest.test_backup_crossproject_user_negative
 cinder_tempest_plugin.api.volume.admin.test_volume_backup.VolumesBackupsTest.test_incremental_backup_respective_parents

How reproducible: always

Steps to Reproduce:
1. Deploy RHOS 16.2
2. Run above mentioned tempest plugin tests
3. Tests fail with the error: "jsonschema.exceptions.ValidationError: None is not of type 'string'"

Actual results:
Traceback (most recent call last):
  File "/usr/lib/python3.6/site-packages/tempest/lib/common/rest_client.py", line 976, in validate_response
    format_checker=FORMAT_CHECKER)
  File "/usr/lib/python3.6/site-packages/jsonschema/validators.py", line 541, in validate
    cls(schema, *args, **kwargs).validate(instance)
  File "/usr/lib/python3.6/site-packages/jsonschema/validators.py", line 130, in validate
    raise error
jsonschema.exceptions.ValidationError: None is not of type 'string'

Failed validating 'type' in schema['properties']['backup']['properties']['name']:
    {'type': 'string'}

On instance['backup']['name']:
    None

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/lib/python3.6/site-packages/cinder_tempest_plugin/api/volume/admin/test_volume_backup.py", line 96, in test_incremental_backup_respective_parents
    volume_id=volume['id'])['backup']
  File "/usr/lib/python3.6/site-packages/tempest/lib/services/volume/v3/backups_client.py", line 38, in create_backup
    self.validate_response(schema.create_backup, resp, body)
  File "/usr/lib/python3.6/site-packages/tempest/lib/common/rest_client.py", line 979, in validate_response
    raise exceptions.InvalidHTTPResponseBody(msg)
tempest.lib.exceptions.InvalidHTTPResponseBody: HTTP response body is invalid json or xml
Details: HTTP response body is invalid (None is not of type 'string'

Failed validating 'type' in schema['properties']['backup']['properties']['name']:
    {'type': 'string'}

On instance['backup']['name']:
    None)

Expected results:
Tests pass

Additional info:
Job and logs info in #c1