Server start/stop run in batches

It seems that virtual router can handle only limited amount of new VMs at the same time, 
so I've limited number of parallel requests so such problems should not emerge.

Quote from API support ticket.
```
So it is possible that the virtual router cannot handle such many VMs (25+) launching at the same time.
Therefore we suggested user to launch VM by no more than 5 at a time.
```