Make volume type optional

Without this patch, creating db instance will fail if no volume type is
provided:

  Validation error: instance['volume']['type'] None is not of type 'string' (HTTP 400)

even if `cinder_volume_type` is configured in trove config file.

Change-Id: I3487e45840f72875ffd6cfb6ca75b4bbf60ab51e

trove/common/apischema.py

@@ -111,7 +111,13 @@ volume = {
     "required": ["size"],
     "properties": {
         "size": volume_size,
-        "type": non_empty_string
+        "type": {
+            "oneOf": [
+                non_empty_string,
+                {"type": "null"}
+            ]
+
+        }
     }
 }