Work around the impending openshift-sdn deprecation in the installer

[danwinship]

The installer will soon reject install-configs that specify networkType: OpenShiftSDN. This changes dev-scripts so that if (a) you're installing 4.15 or later, and (b) you explicitly requested NETWORK_TYPE=OpenShiftSDN, then it will sneak behind the installer's back to give you an openshift-sdn cluster.

(For now, if you implicitly requested sdn by specifying IP_STACK=v4 and no NETWORK_TYPE, then it does nothing special and you will eventually get an error from the installer telling you you should be using ovn-kubernetes. Based on discussion in slack, we will probably change the default for IP_STACK=v4, but not until after everything in CI has been updated to deal with the changed default.)

Not sure about the code style... I had to deal with the fact that (a) network.sh can't call openshift_version, and (b) I can't set a variable in generate_ocp_install_config and then check it from create_cluster, because they get called from separate top-level scripts. So I added an extra variable to network.sh and a helper function to ocp_install_env.sh...

/assign @dtantsur

[danwinship]

/retest

[dtantsur]

@danwinship the image registry revert has merged, could you rebase this change on top of it?

[dtantsur]

/approve

[openshift-ci[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dtantsur

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[dtantsur]

/cc @zaneb @mkowalski

[danwinship]

/test e2e-metal-ipi-ovn-dualstack

[mkowalski]

/lgtm