api-ref: mark id as optional in POST /flavors

When creating a flavor you do not actually have to
provide an id. If one is not provided in the request,
an id is automatically generated from a uuid when
creating the flavor. This updates the api-ref to
point that out.

Change-Id: Ie3c7042bd5f1ae05ad4e4eeba078f82c5e23d33a
Closes-Bug: #1663389

api-ref/source/flavors.inc

@@ -74,7 +74,7 @@ Request
 
    - flavor: flavor
    - name: flavor_name
-   - id: flavor_id_body
+   - id: flavor_id_body_create
    - ram: flavor_ram
    - disk: flavor_disk
    - vcpus: flavor_cpus

api-ref/source/parameters.yaml

@@ -1914,6 +1914,13 @@ flavor_id_body:
   in: body
   required: true
   type: string
+flavor_id_body_create:
+  description: |
+    The ID of the flavor. While people often make this look like an int, this
+    is really a string. If not provided, this defaults to a uuid.
+  in: body
+  required: false
+  type: string
 flavor_is_public:
   description: |
     Whether the flavor is public (available to all projects) or scoped