Make VPN Network configurable

[SimonKienzler]

What this PR does / why we need it:

In gardener/gardener#8987 we proposed to make the VPN network range configurable. Regardless of how exactly this feature is implemented on the Gardener side (e.g. per Shoot or per Seed), the requirements for the vpn2 components stay the same: They need to be able to handle an additional environment variable called VPN_NETWORK and produce a correct VPN config from its value.

This PR introduces some changes to both the seed-server and the shoot-client components to fulfill these requirements.

Which issue(s) this PR fixes:

Part of gardener/gardener#8987

Special notes for your reviewer:

• When the VPN_NETWORK environment variable is not set, the implementation should behave exactly the same as before, ensuring backwards-compatibility.
• A previously implicit requirement (IPv4 networks always have a size of /24) is now explicitly tested, and the script fails if the network has a different size.
• I don't know how or where the shoot-client/path-controller.sh is used, but assumed that it needs to be adapted just as the network-connection.sh scripts.
• From my understanding, the go parts of vpn2 don't need to be altered for this change. Please advise if I missed something there.

Release note:

The VPN components now support configuring a custom VPN network using the VPN_NETWORK environment variable.

[gardener-robot]

@SimonKienzler Thank you for your contribution.

[gardener-robot-ci-3]

Thank you @SimonKienzler for your contribution. Before I can start building your PR, a member of the organization must set the required label(s) {'reviewed/ok-to-test'}. Once started, you can check the build status in the PR checks section below.

[axel7born]

/ok-to-test

[gardener-robot]

@DockToFuture, @marwinski, @ScheererJ, @axel7born You have pull request review open invite, please check

[axel7born]

Hi @SimonKienzler,
thank's for this PR!
Did you test the HA case?
I just tried it and there was no working VPN connection.
I tested without your g/g change so that it should use the default range.
I didn't try to debug the problem yet.

[SimonKienzler]

Thanks for looking into it @axel7born - I'll investigate the HA problems as soon as I can, maybe I missed something.

[SimonKienzler]

Hey @axel7born, we had some priority shifts for our next sprint and I won't be able to spend time working on this feature in the next two weeks. Just wanted to let you know, sorry for any inconvenience. If you find time to debug the troubling HA setup, I'd be happy about any pointers you can provide. Otherwise, we'll pick up the investigation and continue this feature again in a few weeks.

[timebertt]

@SimonKienzler what's the status of this PR? Should we hand this one over to a colleague?

[timebertt]

/assign

[timebertt]

I'm unable to edit the PR description.
Hence, I will open a new PR to simplify things.
/close

[gardener-robot]

@timebertt Command /close is not available to you but only to a Maintainer, Member, Author, Owner.

[timebertt]

@axel7born can you close this PR in favor of #78 and review the new changes there?

[axel7born]

/close this PR in favor of #78