fix: handle destroy with missing security group

[karsten-adot]

This PR makes the following changes:

• Destroying cluser even if security group is missing (for example in case of ungraceful stop during a previous destroy)

I tested this PR by hand and against incomplete cluster (missing security group)

[nchammas]

Hi @karsten-adot and thank you for this fix. I took the liberty of simplifying it a bit given the recent changes we made in #378.

Overall, the code to manage security groups could use a more significant refactoring, but this fix is still helpful.