Use a single Docker registry

[thbkrkr]

For the CI, we use 3 different Docker images: for the operator, the E2E tests and the CI build tools. So far we have tried to use the Docker registry provided by the cloud provider which we are testing as much as possible. With expanding the testing to more providers we have to handle the specifities of each one especially for authentication.

This commit simplifies this by using a single Docker registry for all of our uses.

Resolves #2622.

[david-kow]

Nice work, I think it looks good. The only thing I'd add is removing ACR from AKS setup (from default config, settings and aks.go).

[sebgl]

lgtm

[thbkrkr]

We discussed with the team that there is a lack of good support for the development workflow as pointed in this discussion.

We should still be able to use GCR or a different namespace from that used by the CI in the Elastic Docker registry.

Latest commits address that.

[thbkrkr]

I found several weaknesses in the implemented solution. Please don't do another code review right away.

[thbkrkr]

I think this is ready for a final review.

Since the last review, there is no fundamental change:

• Rename REPOSITORY in NAMESPACE
• Introduce E2E_NAMESPACE and use REGISTRY in E2E_IMG
• Renamings for the propagation of the CI docker creds
• Use IMG_SUFFIX instead of IMG_NAME override
• Add targets to switch between dev and gcr registries
• Move docker login/push logic in a bash script - not ideal but not worse that we had in the Makefile

One weakness that already exists and that remains is that the content of the .registry.env (e.g.: REGISTRY = eu.gcr.io) can be ignored. If the operator image is defined in the .env (e.g.: OPERATOR_IMAGE = docker.elastic.co/eck-snapshots/eck-operator:1.1.0-2020-05-07-7df50bca), it takes the precedence over and this is the one that is used.

[sebgl]

I think overall it's a great improvement.
I did a few tests locally and I left a few comments in the PR.
I'd be OK with merging and fixing things if they break in CI later.