Issue title: Is it possible to transfer proxy to another vds without secret change?
 Issue body: Hello. Is it possible to transfer proxy to another vds without secret change?
 Comments: 
 Comment 0: Yes. I migrate every day. With tha same -p  -H -S -P keys it will be completely identical. You also do not need register new host in the MTProxy Admin Bot. Just set -P key value and enjoy. 
{'source': 'GithubIssues', 'repo_name_with_owner': 'TelegramMessenger/MTProxy', 'issue_id': 333003793, 'repo_stars': 4328.0}