Increase the backup count of mux simulator logs (#5569)

What is the motivation for this PR?
Currently the backup count of mux simulator logs is set to 3. This is not enough for later troubleshooting of mux simulator issues.

How did you do it?
This change increased the backup count from 3 to 15.

Signed-off-by: Xin Wang <xiwang5@microsoft.com>

ansible/roles/vm_set/files/mux_simulator.py

@@ -137,7 +137,7 @@ def config_logging(http_port):
     rfh = RotatingFileHandler(
         '/tmp/mux_simulator_{}.log'.format(http_port),
         maxBytes=10*1024*1024,  # 10MB
-        backupCount=3)
+        backupCount=15)
     fmt = logging.Formatter('%(asctime)s %(levelname)s #%(lineno)d: %(message)s')
     rfh.setFormatter(fmt)
     rfh.setLevel(logging.DEBUG)