Do creation on instance endpoint.

Do not create additional request endpoint, as requesting is like creation, so
reuse POST verb on instance endpoint.

documentation/source/rest.rst

@@ -74,7 +74,7 @@ Request a new instantiation of a software.
 
 `Request`::
 
-  POST http://example.com/api/v1/request HTTP/1.1
+  POST http://example.com/api/v1/instance HTTP/1.1
   Accept: application/json
   Content-Type: application/json; charset=utf-8