MXS-1300: Use native promises for requests

The request-promise-native package adds support for request with native
promises. This is a convenient way to synchronize multiple HTTP requests.

The functions are changed to use promises to make testing easier. With
promises, the testing code can use the chai-as-promised library.

There are a few cases where doRequest is called with a callback that again
calls doAsyncRequest. With multiple hosts, this causes each command to be
propagated to all servers. This is a design flaw of the current multi-host
mode and needs to be changed.

Changed maxctrl.js to wait on the promises to silence some warnings if the
promise rejections are ignored.

maxctrl/lib/call.js

@@ -32,11 +32,11 @@ exports.builder = function(yargs) {
                         }
                     })
 
-                    maxctrl
-                        .doRequest('maxscale/modules/' + argv.module + '/' + argv.command + '?' + argv.parameters.join('&'),
-                                   function(resp) {
-                                       console.log(JSON.stringify(resp, null, 4))
-                                   }, { method: verb })
+                    return maxctrl
+                        .doAsyncRequest('maxscale/modules/' + argv.module + '/' + argv.command + '?' + argv.parameters.join('&'),
+                                        function(resp) {
+                                            console.log(JSON.stringify(resp, null, 4))
+                                        }, { method: verb })
                 })
         })
         .usage('Usage: call <command>')