Fix iptables.flush state: Do not force 'filter' table when flushing

[ghedo]

What does this PR do?

Makes table argument of iptables.flush state work.

The "table" argument is already part of the function signature, this means
that flush() will always force the "filter" table even when the user sets
a different one.

Previous Behavior

table was always set to filter.

New Behavior

table is set to whatever the user passes (or filter if the user doesn't set anything).

Tests written?

No

[cachedout]

Looks like a unit test here may need to be adjusted to confirm with this behavior. Could you please take a look?

[ghedo]

@cachedout ah yes, I didn't realize there were tests for that already. I'll look into it, but it seems to me that commit fd03627 was wrong, since the first argument passed to assertDictEqual()was created with table='' and not table='filter'.

In any case I'll also try to add additional cases to test default and non-default tables to get better coverage.