Kill processes of non-started tarantool servers

It was found that processes of non-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

@@ -1016,16 +1016,6 @@ 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 not silent:
-                raise Exception('Server is not started')
-            else:
-                color_log(
-                    'Server [{0.name}] is not started '
-                    '(status:{0.status}) ...\n'.format(self),
-                    schema='test_var'
-                )
-            return
         if not silent:
             color_stdout('[Instance {}] Stopping the server...\n'.format(
                 self.name), schema='info')