Maintain project dependencies and images

[timuthy]

What this PR does / why we need it:
This PR updates dependency of the druid component.

Along the way, it improves several things:

• Gardener's check script is now executed as part of make check in favor of go vet only (Make target vet has been dropped)
• make revendor now updates .github templates from gardener/gardener
• Several golang-ci findings have been resolved
• The controller-gen from the vendor directory is now considered to generate assets in order to generate a stable output
• Curl has been removed from the Druid image for security reasons
• New leader election options have been added to eventually migrate to lease.coordination.k8s.io based leader election in the future (/cc @timebertt)

⚠️ PR #139 must be merged first!

/cc @shreyas-s-rao @amshuman-kr

I split the PR into different commits to make reviewing easier. HTH.

Release note:

Several Go and image dependencies have been updated.

- [Gardener's check script](https://github.com/gardener/gardener/blob/master/hack/check.sh) is now executed as part of `make check` in favor of `go vet` only (Make target `vet` has been dropped)
- `make revendor` now updates `.github` templates from `gardener/gardener`
- Several golang-ci findings have been resolved
- The `controller-gen` from the `vendor` directory is now considered to generate assets in order to get a stable output.

etcd-druid images won't be delivered with `curl` anymore.

etcd-druid now needs CRUD permissions for `leaese.coordination.k8s.io` object in order to perform leader election.

[timuthy]

ping @gardener/etcd-druid-maintainers

[amshuman-kr]

/lgtm

[amshuman-kr]

Sorry for the delay, @timuthy. I have merged #139. But now this PR has a conflict because I think it has a commit in common with #139. Can you please rebase?

I probably will request you to rebase #146 after this PR is merged because there are common commits there too.

[timuthy]

Thanks @amshuman-kr. I've resolved the merge conflicts and will continue with #146 once this PR is merged.

[amshuman-kr]

Thanks @amshuman-kr. I've resolved the merge conflicts and will continue with #146 once this PR is merged.

Thanks @timuthy! I will try to merge this using rebase instead of merge. Hope that avoids one more rebase for you.

[amshuman-kr]

/lgtm