Adapt terraformer Pod spec to the latest image

[ialidzhikov]

What this PR does / why we need it:
Adapt terraformer Pod spec to the latest image.

Which issue(s) this PR fixes:
Ref gardener-attic/gardener-extensions#597
Part of gardener/gardener#2095

Special notes for your reviewer:
This PR requires:

• terrafomer release - g/terraformer@v1.0.0 released https://github.com/gardener/terraformer/releases/tag/v1.0.0
• Adapt terraformer Pod spec to the latest image gardener-attic/gardener-extensions#624 - merged

Release note:

`nsxt` and `random` providers are now removed from the terraform bundle.

The Terraformer does now lookup the relevant data stored in `ConfigMap`s or `Secret`s live from the system instead of relying on mounted volumes. This is a breaking change as the volume mount approach does no longer work, please adapt your manifests [according to the examples](https://github.com/gardener/terraformer/tree/master/example). The rationale behind it is to not rely on potentially stale kubelet cache while it mounts the volume which may, in rare cases, cause state loss.

`terraformer` does no longer ignore the termination signals sent to PID 1. It does now send a termination signal to the terraform process itself and waits for its completion. This should prevent rare cases in which the `terraformer` was not storing the state of created infrastructure resources.

`g/gardener-extensions` does now require the `g/terraformer@v1.0.0`.

The Packet extension does now support shoot clusters with Kubernetes version 1.18. You should consider the [Kubernetes release notes](https://github.com/kubernetes/kubernetes/blob/master/CHANGELOG-1.18.md) before upgrading to 1.18.

[ialidzhikov]

/hold cancel