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

… in config_db (sonic-net#2847)

#### What I did

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.

#### How I did it

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

#### How to verify it

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

config/main.py

@@ -6631,7 +6631,7 @@ def polling_int(ctx, interval):
     """Set polling-interval for counter-sampling (0 to disable)"""
     if ADHOC_VALIDATION:
         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 = ValidatedConfigDBConnector(ctx.obj['db'])
     sflow_tbl = config_db.get_table('SFLOW')

tests/sflow_test.py

@@ -237,6 +237,7 @@ def test_config_sflow_polling_interval(self):
         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