Add support for passing the tests images registry on Kubevirt Conformance plugin

[ormergi]

What this PR does / why we need it:
Following #6199,
we see errors on the last periodic-kubevirt-push-nightly-conformance job
junit log show that images are being pulled from quay.io/kubevirt instead of the local registry:

Failed to pull image \"quay.io/kubevirt/cirros-container-disk-demo:devel\": rpc error: code = Unknown desc = Error reading manifest devel in quay.io/kubevirt/cirros-container-disk-demo: manifest unknown: manifest unknown"
Kubevirt Conformance test suite pulls images from quay.io/kubevirt by default.

This PR add support for passing DOCKER_PREFIX to the plugin test suite.
It will enable pulling images from other registries including local registry.

This PR also set make conformance to pull from the local registry by default.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Special notes for your reviewer:

Release note:

Kubevirt Conformance plugin now supports passing tests images registry.

[ormergi]

/cc @rmohr @phoracek

[rmohr]

/lgtm
/approve

/hold

@ormergi feel free to unhold when you tested the relevant flows. Code is fine.

[kubevirt-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: rmohr

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

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [rmohr]

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

[ormergi]

@ormergi feel free to unhold when you tested the relevant flows. Code is fine.

Thanks 🙂
I verified the following flows:

• automation/conformance.sh
• make cluster-up cluster-sync conformace

• # from kubevirtci
  make cluster-up
  version=$(curl -L  curl -L https://storage.googleapis.com/kubevirt-prow/devel/nightly/release/kubevirt/kubevirt/latest)
  kubectl apply -f https://storage.googleapis.com/kubevirt-prow/devel/nightly/release/kubevirt/kubevirt/$versionkubevirt-operator.yaml
  kubectl apply -f https://storage.googleapis.com/kubevirt-prow/devel/nightly/release/kubevirt/kubevirt/$version/kubevirt-cr.yaml
  sonobuoy run --wait --e2e-repo-config kubevirtci/hack/confromance-image-config.yaml --plugin https://storage.googleapis.com/kubevirt-prow/devel/nightly/release/kubevirt/$versionconformance.yaml
  

On all flows the correct images were used.

[ormergi]

/unhold

[ormergi]

/test pull-kubevirt-e2e-k8s-1.21-sig-compute

[kubevirt-commenter-bot]

/retest
This bot automatically retries jobs that failed/flaked on approved PRs.
Silence the bot with an /lgtm cancel or /hold comment for consistent failures.