Validate in value before sorting validation

I incorrectly assumed folks wouldn't diverge on the use of the in
value from where it was specified in the OpenAPI spec, but in looking
at the senlin code, I was wrong.

This produces a much more specific warning to explain what is wrong
with the parameters file.

Change-Id: Idffbd55dcd69d8f9ad988f74eb1b4be4b8a56667

os_api_ref/__init__.py

@@ -197,6 +197,17 @@ class RestParametersDirective(Table):
 
         last = None
         for key, value in yaml_data.items():
+            # use of an invalid 'in' value
+            if value['in'] not in sections:
+                self.app.warn(
+                    "``%s`` is not a valid value for 'in' (must be one of: %s)"
+                    ". (see ``%s``)" % (
+                        value['in'],
+                        ", ".join(sorted(sections.keys())),
+                        key)
+                    )
+                continue
+
             if last is None:
                 last = (key, value)
                 continue