[WIP] AGENT-1439: split ove iso build step

[andfasano]

This is a preliminary patch that moves the existing OVE ISO build logic into its own step. The new design will allow to use the step on its own (in a separate CI step); at the same time the step can be also used as part of the main agent workflow, as of now.

[openshift-ci]

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[openshift-ci]

@andfasano: The following tests 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-agent-5control-ipv4	ec9b2ee	link	false	/test e2e-agent-5control-ipv4
ci/prow/e2e-metal-ipi-ovn-ipv6	ec9b2ee	link	true	/test e2e-metal-ipi-ovn-ipv6
ci/prow/e2e-metal-ipi-ovn-dualstack	ec9b2ee	link	false	/test e2e-metal-ipi-ovn-dualstack

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