Don't try to remove server if it does not exist

When a server does not exist anymore (eg if it has been removed manually
before removing it from docker-machine), gracefully handle the situation
and ensure the accompanying SSH-Key is removed aswell.

Fixes: #18

driver.go

@@ -341,12 +341,16 @@ func (d *Driver) Remove() error {
 			return errors.Wrap(err, "could not get server handle")
 		}
 
+		if srv == nil {
+			log.Infof(" -> Server does not exist anymore")
+		} else {
 			log.Infof(" -> Destroying server %s[%d] in...", srv.Name, srv.ID)
 
 			if _, err := d.getClient().Server.Delete(context.Background(), srv); err != nil {
 				return errors.Wrap(err, "could not delete server")
 			}
 		}
+	}
 
 	if !d.IsExistingKey && d.KeyID != 0 {
 		key, err := d.getKey()