This modifies iptables.Exists so that it must be called with an explicit
table and chain. This allows us (a) to generate an appropriate command
line for "iptables -C", which was not previously possible, and (b) it
allows us to limit our strings.Contains() search to just the table and
chain in question, preventing erroneous matches against unrelated rules.
Resolves#10781
Signed-off-by: Lars Kellogg-Stedman <lars@redhat.com>
