[openstack | server] start/stop/pause/suspend actions

2 new requests for openstack compute added:

```ruby
request :start_server
request :stop_server
```

what is more new methods added into openstack compute server model:

```ruby
module Fog
  module Compute
    class OpenStack
      class Server < Fog::Compute::Server
         ...
        def stop ... end
        def pause ... end
        def suspend ... end
        def start ... end
    end
  end
end
```