MXS-1417: Add drain server command to MaxCtrl

The `drain server` commands removes a server from all services and waits
until all the connections for it are closed. Once the server is no longer
in use, it will be set into maintenance mode and put back into the
services where it was removed from.

maxctrl/lib/core.js

@@ -88,6 +88,7 @@ program
     .command(require('./show.js'))
     .command(require('./set.js'))
     .command(require('./clear.js'))
+    .command(require('./drain.js'))
     .command(require('./enable.js'))
     .command(require('./disable.js'))
     .command(require('./create.js'))