Don't remove 'lago' group if its populated

[ifireball]

The lago package currently creates the lago group when it is installed and used need to be added to it.

The package also removes the group when it is removed.

If lago is removed and then re-installed, the groups is removed and then re-created, possibly with a different GID. This causes multiple issues:

1. Users need to be re-added to the group, this can be very annoying because a logoff and logon is required.
2. Files in /var/lib/lago can be left around from the previous installation and become inaccessible to the new installation.

The lago group should only be removed once its confirmed that it not longer contains any users of files.