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/core.js

@@ -82,12 +82,6 @@ module.exports = function(argv) {
             .command('*', 'the default command', {}, () => {
                 console.log('Unknown command. See output of `help` for a list of commands.')
             })
-            .parse(process.argv, function(err, argv, output) {
-                if (err) {
-                    reject(output)
-                } else {
-                    resolve(output);
-                }
-            })
+            .parse(argv, {resolve: resolve, reject: reject})
     })
 }