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/enable.js

@@ -30,7 +30,7 @@ exports.builder = function(yargs) {
                     .updateValue('maxscale/logs', 'data.attributes.parameters.log_' + argv.log, true)
             } else {
                 maxctrl(argv)
-                    .logError('Invalid log priority: ' + argv.log);
+                    .error('Invalid log priority: ' + argv.log);
             }
         })
         .command('maxlog', 'Enable MaxScale logging', {}, function(argv) {