Fix issue: out of range sflow polling interval is accepted and stored…

… in config_db (sonic-net#2847)

Fixed issue: out of range sflow polling interval is accepted and stored in config_db.

Reproduce step:
```
1. Enable sflow feature:   config feature state sflow enabled
2. Enable sflow itself:   config sflow enable
3. Configure out of range polling interval:  config sflow polling-interval 1. Error message is shown as expected
4. Save config:    config save -y
5. Check "SFLOW" section inside config_db
```

As the interval is invalid, the expected behavior is that the interval is not saved to redis. But we see the invalid value was written to redis.

Change `click.echo` to `ctx.fail`

1. Manual test
2. Add a check in existing unit test case to cover the change

config/main.py

@@ -6308,7 +6308,7 @@ def disable(ctx):
 def polling_int(ctx, interval):
     """Set polling-interval for counter-sampling (0 to disable)"""
     if interval not in range(5, 301) and interval != 0:
-        click.echo("Polling interval must be between 5-300 (0 to disable)")
+        ctx.fail("Polling interval must be between 5-300 (0 to disable)")
 
     config_db = ctx.obj['db']
     sflow_tbl = config_db.get_table('SFLOW')

tests/sflow_test.py

@@ -182,6 +182,13 @@ def test_config_sflow_polling_interval(self):
         runner = CliRunner()
         obj = {'db':db.cfgdb}
 
+        # set to 500 out of range
+        result = runner.invoke(config.config.commands["sflow"].
+            commands["polling-interval"], ["500"], obj=obj)
+        print(result.exit_code, result.output)
+        assert result.exit_code != 0
+        assert "Polling interval must be between 5-300" in result.output
+
         # set to 20
         result = runner.invoke(config.config.commands["sflow"].
             commands["polling-interval"], ["20"], obj=obj)