Remove two-stage construction of commands in nwfilter

Remove two-stage construction of commands in nwfilter

The nwfilter ebiptables driver will build up commands to run in
two phases. The first phase contains all of the command, except
for the '-A' part. Instead it has a '%c' placeholder, along with
a '%s' placeholder for a position arg. The second phase than
substitutes these placeholders. The only values ever used for
these substitutions though is '-A' and '', so it is entirely
pointless. Remove the second phase entirely, since it will make
it harder to convert to the new firewall APIs

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>