[config] Several issue records

[LGH-12]

Description

I recorded some issues about config.I hope it is useful.
1、At config/config_mgmt.py:648, there is "Exce" which looks like it should be "Exception".
2、Cmd "config mirror_session erspan add": gre_type is not required. validate_gre_type raise " AttributeError: 'NoneType' object has no attribute 'lower' " if gre_type is not provided.
3、Cmd "config interface cable_length": It is actually "config interface cable-length" at running time and caused by click 7.0 . And It is inconsistent with Command-Reference.
4、Cmd "config route del": In "if not tuple(key.split("|")) in keys and not prefix_tuple in keys", these two look the same.
5、Cmd "config sflow polling-interval": echo error but no return.

Steps to reproduce the issue

1、n/a
2、Run cmd "sudo config mirror_session erspan add TEST 10.10.10.10 10.10.10.11 40 50"
3、Run cmd "config interface -?"
4、n/a
5、Run cmd "config sflow polling-interval 301" and "show sflow"

Describe the results you received

Describe the results you expected

Additional information you deem important (e.g. issue happens only occasionally)

Output of show version

(paste your output here)

[LGH-12]

I have a pr with fix. Please help in review that.

[LGH-12]

Hi @bingwang-ms . I noticed you fixed the gre_type in #2458. I have removed that in my PR. Could you please help to review the remaining code or assign someone?