tests: fix flaky tests in test_running_cluster.py

The problem occurs when trying to stop cluster (either with 'tt stop' or
with 'tt restart') at the moment bootstrap have not been completed.
In such a scenario when the replica instance is stopped with either of
the mentioned commands the corresponding master instance exits instantly
with the critical failure as below:

F> can't initialize storage: Can't check who replica <...> at <...> chose its bootstrap leader

As a result 'tt stop/restart' for the master instance doesn't produce the
termination-message:

The Instance small_cluster_app:storage-master (PID = <...>) has been terminated

because such kind of message is produced only for a running instance.

This commit introduces waiting for the cluster bootstrap completion
after it is started or restarted.

Closes #1000

test/integration/running/test_running_cluster.py

@@ -8,7 +8,7 @@
 
 from utils import (control_socket, extract_status, get_tarantool_version,
                    lib_path, log_path, run_command_and_get_output, run_path,
-                   wait_files, wait_pid_disappear)
+                   wait_event, wait_files, wait_pid_disappear)
 
 tarantool_major_version, tarantool_minor_version = get_tarantool_version()
 
@@ -45,6 +45,18 @@ def wait_cluster_started(tt_cmd, workdir, app_name, instances, inst_conf):
         files.append(conf['console_socket'])
     assert wait_files(5, files)
 
+    def are_all_box_statuses_running():
+        status_cmd = [tt_cmd, "status", app_name]
+        status_rc, status_out = run_command_and_get_output(status_cmd, cwd=workdir)
+        assert status_rc == 0
+        status_info = extract_status(status_out)
+        for inst in instances:
+            inst_id = app_name + ":" + inst
+            if status_info[inst_id].get("BOX") != "running":
+                return False
+        return True
+    assert wait_event(5, are_all_box_statuses_running)
+
 
 def default_inst_conf(workdir, app_name, inst):
     app_path = os.path.join(workdir, app_name)