create image without pre-installed tf plugins

[Diaphteiros]

How to categorize this PR?

/kind enhancement
/priority normal

What this PR does / why we need it:
As discussed in #74, we would like a terraformer image without any pre-installed terraform plugins.

Which issue(s) this PR fixes:
Fixes #74

Special notes for your reviewer:
Not sure whether terraformer-none or terraformer-empty is the more intuitive name here. WDYT?

Release note:

Terraformer will now publish an additional image without any pre-installed terraform plugins.

[Diaphteiros]

cc @schrodit @timebertt

[rfranzke]

I think you have to extend https://github.com/gardener/terraformer/blob/master/.ci/pipeline_definitions#L10-L50

[timebertt]

Unsure about the naming, rest lgtm

[timebertt]

/squash

[Diaphteiros]

@timebertt I renamed the image to terraformer-slim, but I kept the none in the Makefile - is this ok or should I rename it there too?
Should I squash this PR into one commit or will you do it during merging?

[timebertt]

Yes, please rename it everywhere consistently to avoid confusion.

Should I squash this PR into one commit or will you do it during merging?

I've already added the merge/squash label which indicates that the person merging the PR should use the "Squash and Merge" option.

[Diaphteiros]

Yes, please rename it everywhere consistently to avoid confusion.

Done.

[timebertt]

/lgtm

[timebertt]

Image build fails: https://concourse.ci.gardener.cloud/teams/gardener/pipelines/terraformer-master/jobs/master-head-update-job/builds/23#L603545bb:206:209

Step 22/24 : COPY --from=terraform-base /tmp/terraformer/terraform /bin/terraform
 ---> f40151d11bff
Step 23/24 : COPY --from=terraform-base /tmp/terraformer/terraform-provider* /terraform-providers/
COPY failed: no source files were specified

@Diaphteiros can you please fix it and check that it works locally using docker build (buildkit seems to not fail builds if COPY has no source files)?

[Diaphteiros]

@timebertt I just tried it locally, but make docker-images as well as docker build both run through without any problems on my system. I will have a look at the pipeline.

[Diaphteiros]

Ah, it's the publish step that fails. Will check that.