Allow overriding OPENSHIFT_RELEASE_TAG

[dtantsur]

The current auto-detection logic does not work at all when quay.io
images are used (a custom payload).

[openshift-ci]

[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 ask for approval from dtantsur. 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

[elfosardo]

/lgtm

[openshift-ci]

@dtantsur: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ci/prow/e2e-metal-ipi-serial-ipv4	3e6083d	link	true	/test e2e-metal-ipi-serial-ipv4

Full PR test history. Your PR dashboard.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository. I understand the commands that are listed here.

[dtantsur]

/retest-required