[route_check]Add an option for route_check to control whether log is writen to syslog

[bingwang-ms]

Fix sonic-net/sonic-buildimage#5804
- What I did
By default, route_check.py will report an ERROR in syslog if route mismatch is detected, which is out of the control of monit config file and cause LogAnalyzer to report test error. This commit add an option (-s) to control this behavior. And log is not writen to syslog in default.

- How I did it
Add an option (-s) for route_checker.py.

- How to verify it
The update is verified on Arista-7260.

1. Add a static route whose nexthop is not reachable.

ip route add 1.1.1.1 via 192.168.1.101

2. Run route_check.py, and error msg is only printed on stdout. Nothing is writen to syslog
3. Run route_check.py -s. and error msg is writen to both stdout and syslog
4. Wait for 15 minutes, and confirm that monit will report the error

Nov  4 09:30:36.917367 str-7260cx3-acs-2 ERR monit[631]: 'routeCheck' status failed (255) -- results: { {#012    "missed_ROUTE_TABLE_routes": [#012        "1.1.1.1/32"#012    ]#012} }#012 Failed. Look at reported mismatches above

- Previous command output (if the output of a command-line utility has changed)
Output is not changed.

- New command output (if the output of a command-line utility has changed)
Output is not changed.

[renukamanavalan]

Thank you!