firewall: support ingressPolicy=(open|same-bridge) for isolating bridges as in Docker

This commit adds a new parameter `ingressPolicy` (`string`) to the `firewall` plugin.
The supported values are `open` and `same-bridge`.

- `open` is the default and does NOP.

- `same-bridge` creates "CNI-ISOLATION-STAGE-1" and "CNI-ISOLATION-STAGE-2"
that are similar to Docker libnetwork's "DOCKER-ISOLATION-STAGE-1" and
"DOCKER-ISOLATION-STAGE-2" rules.

e.g., when `ns1` and `ns2` are connected to bridge `cni1`, and `ns3` is
connected to bridge `cni2`, the `same-bridge` ingress policy disallows
communications between `ns1` and `ns3`, while allowing communications
between `ns1` and `ns2`.

Please refer to the comment lines in `ingresspolicy.go` for the actual iptables rules.

The `same-bridge` ingress policy is expected to be used in conjunction
with `bridge` plugin. May not work as expected with other "main" plugins.

It should be also noted that the `same-bridge` ingress policy executes
raw `iptables` commands directly, even when the `backend` is set to `firewalld`.
We could potentially use the "direct" API of firewalld [1] to execute
iptables via firewalld, but it doesn't seem to have a clear benefit over just directly
executing raw iptables commands.
(Anyway, we have been already executing raw iptables commands in the `portmap` plugin)

[1] https://firewalld.org/documentation/direct/options.html

This commit replaces the `isolation` plugin proposal (issue 573, PR 574).
The design of `ingressPolicy` was discussed in the comments of the withdrawn PR 574 ,
but `same-network` was renamed to `same-bridge` then.

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

pkg/utils/iptables.go

@@ -119,3 +119,21 @@ func ClearChain(ipt *iptables.IPTables, table, chain string) error {
 		return err
 	}
 }
+
+// InsertUnique will add a rule to a chain if it does not already exist.
+// By default the rule is appended, unless prepend is true.
+func InsertUnique(ipt *iptables.IPTables, table, chain string, prepend bool, rule []string) error {
+	exists, err := ipt.Exists(table, chain, rule...)
+	if err != nil {
+		return err
+	}
+	if exists {
+		return nil
+	}
+
+	if prepend {
+		return ipt.Insert(table, chain, 1, rule...)
+	} else {
+		return ipt.Append(table, chain, rule...)
+	}
+}