use '--kube-version' flag for helm template

[Diaphteiros]

What this PR does / why we need it:

The helm template command takes an optional --kube-version flag, where the cluster's kubernetes version can be provided. This allows helm charts to differentiate between different k8s versions by using .Capabilities.KubeVersion.
With this PR, the --kube-version flag is provided to all helm template calls.

Release note:

garden-setup will now provide the cluster's version to `helm template` calls via the `--kube-version` flag. This has a few side effects:
- There is a new component called `k8sversion`. It's a dummy component, which doesn't actually deploy anything, but it fetches each pre-existing cluster's (= base cluster and potential soils) k8s version instead.
- After deployment of the virtual kube-apiserver, the export generation now waits until it is ready, so it can fetch its k8s version.
- ⚠️ Since helm might deploy different manifests for different k8s versions, this change means that the result of `sow deploy` now also depends on the k8s versions of all clusters in `landscape.iaas`.
  - Re-running `sow deploy` after upgrading the base cluster to a higher k8s version could therefore lead to changes.
  - This could be prevented by not re-running the `k8sversion` component, as that one is responsible for fetching the k8s versions. Most of the time, deploying the helm chart fitting to the cluster is probably desired, though.