[master] Fix 64260: Remove isfile check to increase speed of listing large numbers of keys

[bluesliverx]

What does this PR do?

It adds a configuration option for skipping the isfile check when enumerating keys in various situations. This speeds up minion key listing significantly in certain situations.

What issues does this PR fix or reference?

Fixes: #64260

Previous Behavior

Minion key listing and salt master startup took minutes with 10k+ minions and key storage on a slow filesystem (e.g. NAS).

New Behavior

Minion key listing is performant.

Merge requirements satisfied?

[NOTICE] Bug fixes or features added to Salt require tests.

• Docs
• Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html
• Tests written/updated

Commits signed with GPG?

No

[Ch3LL]

Looks good, but I would also like to get @dwoz 's review here

[dwoz]

I'm not sure I agree with adding an option for this. I think it would make sense to have this (not checking isfile) as the default. In general, I think we should default to the more performant case and only add an option for the less performant case when absolutely necessary. In this particular case, I can't see a reason for the less performant state. Let's assume salt is the only thing managing the keys and in the rare situation we run into a bad one, warn, clean it up, and move on.