Previously, if a user or role was removed from
`credentials.users` or `credentials.roles` in config,
it still remained in the instance after reload. This was
intentional to prevent bricking when an empty `credentials`
section was rolled out, but it left stale accounts and roles
in the system, which is a security risk.

Now both users and roles are synchronized with config on reload:
 - users/roles missing from config are dropped,
 - users/roles present in config are created/updated,
 - manually created users/roles remain untouched.

Closes tarantool#11827

NO_DOC=bugfix