Potential risk of acl_loader missing ctx in main function

[inspurSDN]

Issue: Potential risk about cli() function without ctx in main function of acl_loader

Root cause: SONiC will call cli() directly in acl-loader. But it still exist potential risk when run main in acl-loader

Solution: Remover main funcion which missing ctx object and remain cli() function for acl-loader call directly

[inspurSDN]

@lguohan could you help to review it?

[lguohan]

@qiluo-msft , can you check this pr?