Support for Firewalld

Firewalld [1] is a firewall managing daemon with D-Bus interface.

What sort of problem are we trying to solve with this ?

Firewalld internally also executes iptables/ip6tables to change firewall settings.
It might happen on systems where both docker and firewalld are running
concurrently, that both of them try to call iptables at the same time.
The result is that the second one fails because the first one is holding a xtables lock.
One workaround is to use --wait/-w option in both
docker & firewalld when calling iptables.
It's already been done in both upstreams:
https://github.com/docker/docker/commit/b315c380f4acd65cc0428009702f99a266f96c59
https://github.com/t-woerner/firewalld/commit/b3b451d6f8946986b8f50c8bcddeef50ed7a5f8f
But it'd still be better if docker used firewalld when it's running.

Other problem the firewalld support would solve is that
iptables/firewalld service's restart flushes all firewall rules
previously added by docker.
See next patch for possible solution.

This patch utilizes firewalld's D-Bus interface.
If firewalld is running, we call direct.passthrough() [2] method instead
of executing iptables directly.
direct.passthrough() takes the same arguments as iptables tool itself
and passes them through to iptables tool.
It might be better to use other methods, like direct.addChain and
direct.addRule [3] so it'd be more intergrated with firewalld, but
that'd make the patch much bigger.
If firewalld is not running, everything works as before.

[1] http://www.firewalld.org/
[2] https://jpopelka.fedorapeople.org/firewalld/doc/firewalld.dbus.html#FirewallD1.direct.Methods.passthrough
[3] https://jpopelka.fedorapeople.org/firewalld/doc/firewalld.dbus.html#FirewallD1.direct.Methods.addChain
    https://jpopelka.fedorapeople.org/firewalld/doc/firewalld.dbus.html#FirewallD1.direct.Methods.addRule

Signed-off-by: Jiri Popelka <jpopelka@redhat.com>

daemon/networkdriver/bridge/driver.go

@@ -222,6 +222,10 @@ func InitDriver(config *Config) error {
 		bridgeIPv6Addr = networkv6.IP
 	}
 
+	if config.EnableIptables {
+		iptables.FirewalldInit()
+	}
+
 	// Configure iptables for link support
 	if config.EnableIptables {
 		if err := setupIPTables(addrv4, config.InterContainerCommunication, config.EnableIpMasq); err != nil {