Add ploop in disk_format

"ploop" image format is supported in upstream Glance
https://review.openstack.org/341633

And similar patch has been added in python-openstackclient:
https://review.openstack.org/411405

Co-Authored-By: yuyafei <yu.yafei@zte.com.cn>
Change-Id: I1471224df97cf5fecfe7f02e549855af81c45848
Related-Bug: 1650342

glanceclient/tests/unit/v2/test_shell_v2.py

@@ -46,7 +46,7 @@ def schema_args(schema_getter, omit=None):
                 'description': 'Format of the container'},
             'disk_format': {
                 'enum': [None, 'ami', 'ari', 'aki', 'vhd', 'vhdx', 'vmdk',
-                         'raw', 'qcow2', 'vdi', 'iso'],
+                         'raw', 'qcow2', 'vdi', 'iso', 'ploop'],
                 'type': 'string',
                 'description': 'Format of the disk'},
             'location': {'type': 'string'},