Add signal argument to server restart routine

Found that stop and restart routines should have the ability to set
signal using restart routine to fix the issues, like:

Test hung! Result content mismatch:
--- replication-py/init_storage.result	Wed Aug 26 06:06:15 2020
+++ /tmp/tnt/101_replication-py/init_storage.result	Wed Nov 11
10:17:50 2020
@@ -130,5 +130,3 @@
 -------------------------------------------------------------
 waiting reconnect on JOIN...
 ok
-waiting reconnect on SUBSCRIBE...
-ok

Author: avtikhon

lib/tarantool_server.py

@@ -986,8 +986,8 @@ def stop(self, silent=True, signal=signal.SIGTERM):
             if os.path.exists(self._admin.port):
                 os.unlink(self._admin.port)
 
-    def restart(self):
-        self.stop()
+    def restart(self, signal=signal.SIGTERM):
+        self.stop(signal)
         self.start()
 
     def kill_old_server(self, silent=True):