gp: Support more global trust directories

gp: Support more global trust directories

In addition to the SUSE global trust directory, add support for RHEL and
Debian-based distributions (including Ubuntu).

To determine the correct directory to use, we iterate over the variants
and stop at the first which is a directory.

In case none is found, fallback to the first option which will produce a
warning as it did previously.

Signed-off-by: Gabriel Nagy <gabriel.nagy@canonical.com>
Reviewed-by: Joseph Sutton <josephsutton@catalyst.net.nz>
Reviewed-by: David Mulder <dmulder@samba.org>