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/unlink.js

@@ -25,8 +25,8 @@ function removeServer(argv, path, targets) {
             _.set(res, 'data.relationships.servers.data', servers)
             delete res.data.attributes
 
-            maxctrl(argv)
-                .doRequest(path, null, {method: 'PATCH', body: res})
+            return maxctrl(argv)
+                .doAsyncRequest(path, null, {method: 'PATCH', body: res})
         })
 }