create iptables forwarding rule targeting containerlab nodes

[hellt]

fix #781

iptables rules management have been added to docker runtime only so far.

Note, apart from the changes done in iptables.go file where the funcs to add/remove iptables rules are, the following changes have been made:

1. deploy and destroy cmd functions have been scoped out to deployFn and destroyFn accordingly.
2. Mgmt() method has been added to Runtime interface to allow pulling the management struct
3. Runtime's struct Mgmt field was changed to mgmt

[hellt]

@LimeHat when you create a management network in podman you don't save the bridge interface under c.mgmt.Bridge like we do for docker - https://github.com/srl-labs/containerlab/blob/iptables-fwd/runtime/docker/docker.go#L204?

Is it feasible to add this step so that this PR can have podman support? The idea here is to automatically add iptables rule allowing external traffic to reach the containerlab nodes. This is achieved by installing an allow rule in the FORWARD chain.

[hellt]

Let me merge this one, and podman we can address in a separate PR. At this moment the iptables management will only happen if the global runtime is set to docker