Support forcing CNI network creation even if delete of pre-existing network fails

[sipsma]

In reference to the current code's attempt to delete a pre-existing network for a given VM, I've now encountered in the wild a plugin (the firewall CNI plugin) that consistently fails here due to a previous result not being available (which goes against what the CNI spec says).

While we can fix this problem upstream, we should probably also include the ability for users to optionally override failing at that line (perhaps via something like a Force parameter in the CNI configuration) in order to allow use of CNI plugins that aren't perfectly compliant. The default behavior should stay the same though and if the error is overridden with the Force parameter then the error needs to be logged still.

[Zyqsempai]

I will work on this one!

[sipsma]

I will work on this one!

Sounds great, thanks! Let me know if you need any more context on the change

[sipsma]

Closed by #161