Bug 14532: (followup) Use tar's --exclude for simplicity

Bug 14532: (followup) Use tar's --exclude for simplicity

This patch creates an --exclude definition for the tar command
so it is easier to stack excluded stuff. It does so for the
authority and biblio indexes if --exclude-indexes is passed.

A side effect from this, is that uploads and plugins are still
backed up, as Jonathan noted.

Regards

Edit: Fixed a weird quoting problem.

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Brendan Gallagher brendan@bywatersolutions.com