make keystone debug logging available separately

[James Troup]

This is obviously a wishlist type bug. Many times we find ourselves
in a situation where we have to turn on DEBUG level logging in
Keystone in order to be able to resolve an issue. Unfortunately DEBUG
level logging is noisy enough that we can't just leave it enabled all
the time.

One possible solution to this would be to enable DEBUG level logging
but to have it logged to a separate file/directory from the normal
keystone logs. This would allow a) it to be much more aggressively
rotated (e.g. capped by size) and b) not to be sent to a central log
aggregator like the normal logs are.

It would be nice if the charms had an option/action to allow this.