improve(ws): add controller-image parameter to e2e tests

[etirelli]

Adding support for a command line parameter to set which image to use for e2e tests.

To run the tests with a given image, use the IMG parameter:

make test-e2e IMG=<some-registry>/workspace-controller:tag

Please note that if not defined, the default for e2e tests is now:

IMG ?= controller:latest

[google-oss-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by:
Once this PR has been reviewed and has the lgtm label, please assign kimwnasptd for approval. For more information see the Kubernetes Code Review Process.

The full list of commands accepted by this bot can be found here.

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[etirelli]

I was not sure if I should change the default here to ghcr.io/kubeflow/notebooks/workspace-controller:latest to stay consistent with the previous e2e test behaviour or even create a separate parameter for it. Let me know please if you prefer that I do that.

[ederign]

/assign @thesuperzapper