MXS-1220: Add output for module commands

The module commands can now produce JSON formatted output which is passed
to the caller. The output should conform to the JSON API as closely as
possible.

Currently, the REST API wraps all JSON produced by module commands inside
a meta-object of the following type:

  {
    "meta": <output of module command>
  }

This allows the output to be JSON API conformant without modifying the
modules and allows incremental updates to code.

include/maxscale/modulecmd.h

@@ -29,6 +29,7 @@
 #include <maxscale/server.h>
 #include <maxscale/service.h>
 #include <maxscale/session.h>
+#include <maxscale/jansson.h>
 
 MXS_BEGIN_DECLS
 
@@ -118,10 +119,20 @@ typedef struct
  * the argument was not provided, the type of the argument will be
  * MODULECMD_ARG_NONE.
  *
- * @param argv Argument list
+ * If the module command produces output, it should be stored in the @c output
+ * parameter as a json_t pointer. The output should conform as closely as possible
+ * to the JSON API specification. The minimal requirement for a JSON API conforming
+ * object is that it has a `meta` field. Ideally, the `meta` field should not
+ * be used as it offloads the work to the client.
+ *
+ * @see http://jsonapi.org/format/
+ *
+ * @param argv   Argument list
+ * @param output JSON formatted output from the command
+ *
  * @return True on success, false on error
  */
-typedef bool (*MODULECMDFN)(const MODULECMD_ARG *argv);
+typedef bool (*MODULECMDFN)(const MODULECMD_ARG *argv, json_t** output);
 
 /**
  * A registered command
@@ -224,11 +235,13 @@ bool modulecmd_requires_output_dcb(const MODULECMD* cmd);
  * on the length of the call or whether it will block. All of this depends on the
  * module and what the command does.
  *
- * @param cmd Command to call
- * @param args Parsed command arguments
+ * @param cmd    Command to call
+ * @param args   Parsed command arguments, pass NULL for no arguments
+ * @param output JSON output of the called command, pass NULL to ignore output
+ *
  * @return True on success, false on error
  */
-bool modulecmd_call_command(const MODULECMD *cmd, const MODULECMD_ARG *args);
+bool modulecmd_call_command(const MODULECMD *cmd, const MODULECMD_ARG *args, json_t** output);
 
 /**
  * @brief Set the current error message