fix #4204: automatically update usages of group when it is renamed

fix #4204: automatically update usages of group when it is renamed

When renaming a group, the usages didn't get updated automatically. To
get around problems with atomicity, the old rule is first cloned with the
new name, the usages are updated and only when updating has finished, the
old rule is deleted.

The subroutines that lock/update host configs had to be changed so that
it's possible to lock any config, not just the one of the current host.

Signed-off-by: Leo Nunner <l.nunner@proxmox.com>