✨ allow multiple subnets if first matches the configured one on HetznerCluster

[fgrutsch]

What this PR does / why we need it:

We are currently running a setup where we have a k8s cluster running on Hetnzer cloud with a public IP and a private network. We also have bare metal nodes (setup manually and not running k8s) which we are connecting via a private network (vSwitch) to the k8s nodes. The setup works in general, but the controller reports errors due to multiple subnets restriction.

The idea of the PR is that we could allow multiple subnets in a network as long as the first subnet matches with the one that is configured on the HetznerCluster resource. When attaching a server to the network it will use the first subnet by default.

Wdyt about that solution? I have seen in multiple issues that you don't use private networks in your product and I think therefore this would be an acceptable solution to connect bare metal nodes <-> cloud nodes without making major modifications to the project's code.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):

• Private network for bare metal host / cloud hybrid #213
• Support adding vSwitch subnets to the network #1493

Special notes for your reviewer:

TODOs:

• squash commits
• include documentation
• add unit tests