[Feature] Enhanced k3d-proxy configuration

[iwilltry42]

This is a preparation to more easily allow for updating port mappings on a running cluster.
More details will follow.

Preliminary & Incomplete Changelog

• k3d-proxy / serverlb
  • updated confd template for nginx, now based on a yaml file injected just before container startup (or at runtime)
  • updated confd version (custom fork) to use -watch so we don't need to issue confd -onetime and nginx -s reload anymore when updating the config
• revamped loadbalancer creation + update procedure
• NodeHooks are now part of the Node type struct, so they don't get lost underways
• defaults for helper images + ability to set them from env separately

To-Dos

• Properly update the loadbalancer when new nodes are added (as we have it now)
• adapt new node edit --port-add functionality to new loadbalancer config file
• when nodes are added and the lb config updated, ensure that the lb is still fine (could break due to broken upstreams)

Follow-Up Issues

• NodeDelete: success message at the end (i.e. also after updating the LB)
• NodeCreate/NodeDelete: single collective update of the loadbalancer
  • only, when nodes are up and running