Implement graceful shutdown

[alexandrebrg]

What needs to be done

• Wait for a SIGKILL/SIGINT signal is received to trigger graceful shutdown¹
• Every runtime / component must have a method / capacity to kill what it has created (containers, VMs, iptables...)
• Graceful shutdown should wait for all components to end their shutdown

Why it is needed

• Riklet shutdown is not killing all containers / VMs created
• Network management isn't deleting iptables on shutdown

Footnotes

1. Learn more on tokio docs ↩

[alexandrebrg]

Hi @kalil-pelissier, the work done on instance deletion should be helpful for you as it gives you all the necessary methods to clean up workloads (#84)!