exoscale: Rework EIP access from workers (#7337)

Context: Load-balancing in Exoscale is performed by associating many
workers with the same EIP. This works, however, the workers cannot access
themselves via the EIP, which is needed at least for cert-managers
"self-test".

Problem: The old iptables based workaround felt fragile and disappointed
me at least once.

New solution: Add the EIP to a loopback interface on each worker.