oo-accept-systems: Fix check_nodes_cartridges

Fix the check_nodes_cartridges test in oo-accept-systems to get the list
of cartridges the correct way rather than trying to use the "cart_list"
fact, which has been deprecated for several releases.

With commit 4e94c47a381f3cfd168e9add0383414202aa96e5, the cart_list fact
is no longer updated, so the check was essentially a no-op (no nodes
would respond with the fact, so the carts_for Hash would remain empty,
which would cause the test to terminate silently).

This commit fixes bug 1118908.
oo-accept-systems: Better output in carts check

Improve the output in the check_nodes_cartridges check of
oo-accept-systems: Sort cartridges before printing in order to make it
easier to compare lists, and use consistent formatting.