Implement --keep-mgmt-net for destroy subcmd

[steiler]

Implementing #271.

This is based upon changes introduced in #460.

At the same time it adds an implementation of the DeleteNet() for containerd.
Here we are listing all the Host interfaces to see if the bridge is still the master to any existing device, if not, and not --keep-mgmt-net is set, the deletion of the cni based mgmt bridge is executed.

[steiler]

Alright then, I'd consider this done now.

[steiler]

Resolved merge conflicts.
We're ready to merge here, aren't we?

[hellt]

We're ready to merge here, aren't we?

I think it is time to add a test for this case

[steiler]

@hellt are you fine with the tests

[hellt]

@steiler can you please also add a doc entry about this flag of destroy command?

[hellt]

@steiler also I think you wanted to add the schema entry for bridge under mgmt?

[steiler]

@hellt That is not related to this flag, so it should go into a new PR.

[steiler]

Since we've gone downa different path now, with karims work on refactoring the runtime interface, should the withKeepMgmtNet() func now be replaced by jsut the field in the withConfig piece?
https://github.com/srl-labs/containerlab/pull/462/files#diff-bb8ad382f59edfae9297e446895a161036f39115189e67da9d9f377a684a8f82R28

[hellt]

@steiler

Since we've gone downa different path now, with karims work on refactoring the runtime interface, should the withKeepMgmtNet() func now be replaced by jsut the field in the withConfig piece?

can you create a commit with that approach? Looks more clean to me

[steiler]

As discussed, we go with this approach for now and tackle a potential change in a different PR.