Kill hanging processes of not started servers

It was found that hanging processes of not started tarantool servers
are not killed by test-run and leave to hang. This situation can be
reproduced by creating the main server, then creating a replica server,
but the replica server is unable to join the master, for example, due
to lack of user permissions. In this case, the test fails by the server
start timeout and test-run kills the main server process only.
This patch fixes the issue.

Fixes #256
Follows up #276

lib/tarantool_server.py

@@ -1016,7 +1016,7 @@ def stop(self, silent=True, signal=signal.SIGTERM):
             color_log('Server [%s] start against running ...\n',
                       schema='test_var')
             return
-        if self.status != 'started':
+        if self.status != 'started' and not hasattr(self, 'process'):
             if not silent:
                 raise Exception('Server is not started')
             else: