Add docker to image_schema on glance v2 cli

Add docker to v2 image_schema
Add docker to v2 unit tests

This is related to following glance api extension.
  https://review.openstack.org/#/c/249282/

Co-Authored-By: Kairat Kushaev <kkushaev@mirantis.com>

Closes-Bug: #1519402
Change-Id: Ia015f027788b49c1b0002fb3e3a93ac825854596

glanceclient/v2/image_schema.py

@@ -44,7 +44,8 @@ _BASE_SCHEMA = {
                 "aki",
                 "bare",
                 "ovf",
-                "ova"
+                "ova",
+                "docker"
             ],
             "type": "string",
             "description": "Format of the container"