Add login-prefix config option for created logins

[gagbo]

Hello,

First, thanks for ktra, it looks like it covers my needs to get a quickly setup for a private registry !

Since there may be a few things that I wanted to tweak (possibly looking into OAuth if I get the courage), I tried to force myself to look into the code by implementing a small feature: I noticed that there is a prefix before the username for all DB related operations, and it was hard coded. I thought it'd be great if it were possible to customize the user_login prefix as well to make ktra feel more "personal". I had to type cargo owners --registry ktra --add ktra-secure-auth:gagbo to add an owner, so I thought changing the prefix would be nice as it ends up in the API the users have to use.

The added config option goes under the db_config section, and defaults to the previously hardcoded option ktra-secure-auth: if absent: so the change is backwards compatible.

Feel free to discard it if you think it's unnecessary, I didn't really create an issue or discussed this point before coming with a patch.

Hopefully I can try to help a little with #28. We're all busy but if we can each give a little time it would probably work out 🤞

Regards,
Gerry