Update openapi.yaml, volume label required

The volume label is required according to the behavior of the API and the intent matches this behavior because the source code agrees with the label being a required field upon volume creation.

Author: abemassry

openapi.yaml

@@ -15170,6 +15170,7 @@ paths:
                   minLength: 1
                   maxLength: 32
                   pattern: '^[a-zA-Z]((?!--|__)[a-zA-Z0-9-_])+$'
+                  required: true
                 config_id:
                   type: integer
                   description: >