Fix codegen problems in swagger spec

Code generation does't support multiple types

Signed-off-by: Daniel Nephin <dnephin@docker.com>

api/swagger.yaml

@@ -254,6 +254,7 @@ definitions:
         properties:
           Propagation:
             description: "A propagation mode with the value `[r]private`, `[r]shared`, or `[r]slave`."
+            type: "string"
             enum:
               - "private"
               - "rprivate"
@@ -823,9 +824,7 @@ definitions:
           type: "string"
       Cmd:
         description: "Command to run specified as a string or an array of strings."
-        type:
-          - "array"
-          - "string"
+        type: "array"
         items:
           type: "string"
       Healthcheck:
@@ -853,9 +852,7 @@ definitions:
           The entry point for the container as a string or an array of strings.
 
           If the array consists of exactly one empty string (`[""]`) then the entry point is reset to system default (i.e., the entry point used by docker when there is no `ENTRYPOINT` instruction in the `Dockerfile`).
-        type:
-          - "array"
-          - "string"
+        type: "array"
         items:
           type: "string"
       NetworkDisabled: