[master] Terraformer v2

[timebertt]

What this PR does / why we need it:

The terraformer v2 rewrite is finished and all open tasks have been completed.
So this PR merges terraformer-v2 onto master (without any changes, only resolving merge conflicts).

Which issue(s) this PR fixes:
Fixes #40

Special notes for your reviewer:

Release note:

Terraformer version `v2` now requires to pass in ConfigMap and Secret names via command line flags instead of environment variables and the base command of the docker image has changed to `/terraformer`. Please see the [examples](https://github.com/gardener/terraformer/tree/master/example) for more details.

If your provider extension is deploying terraformer Pods via gardener's [terraformer library](https://github.com/gardener/gardener/tree/master/extensions/pkg/terraformer), please make sure, that you use at least `gardener/gardener@v1.12.0` and set `terraformer.UseV2(true)` in order to deploy a PodSpec, that is compatible with terraformer `v2`.

Terraformer was rewritten in go and now watches the terraform state file in order to continuously update the state ConfigMap to not lose any relevant infrastructure state.

You can use `make start-dev-container` to start a docker container which can run terraformer and tests in an isolated environment for development and testing. Also `make start` will run terraformer commands in such a development container.

[rfranzke]

/lgtm

[timebertt]

/test

[testmachinery]

Testrun: e2e-gwgvj
Workflow: e2e-gwgvj-wf
Phase: Succeeded

+--------------+--------------+-----------+----------+
|     NAME     |     STEP     |   PHASE   | DURATION |
+--------------+--------------+-----------+----------+
| pod-e2e-test | pod-e2e-test | Succeeded | 2m34s    |
+--------------+--------------+-----------+----------+