MXS-1220: Add more validation checks to request JSON

The requests that send a body should define at least a `data` member.

Added a simple test that checks that bad requests are rejected. This test
should be expanded to check that the returned error body contains the
correct members.

include/maxscale/config.h

@@ -37,6 +37,7 @@ MXS_BEGIN_DECLS
 #define MAX_ADMIN_HOST_LEN      1024
 
 /** JSON Pointers to key parts of JSON objects */
+#define MXS_JSON_PTR_DATA       "/data"
 #define MXS_JSON_PTR_ID         "/data/id"
 #define MXS_JSON_PTR_PARAMETERS "/data/attributes/parameters"