OPNET-748: Support running with external loadbalancer

[cybertron]

Add an EXTERNAL_LOADBALANCER option to the config settings that configures the external loadbalancer and tells the cluster to use it instead of the internal one.

[openshift-ci]

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[openshift-ci]

@cybertron: 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/images	f43725b	link	true	/test images
ci/prow/e2e-agent-compact-ipv4	f43725b	link	true	/test e2e-agent-compact-ipv4
ci/prow/e2e-agent-5control-ipv4	f43725b	link	false	/test e2e-agent-5control-ipv4
ci/prow/e2e-agent-compact-ipv4-iso-no-registry	f43725b	link	false	/test e2e-agent-compact-ipv4-iso-no-registry
ci/prow/e2e-agent-4control-ipv4	f43725b	link	false	/test e2e-agent-4control-ipv4
ci/prow/e2e-agent-ha-dualstack	f43725b	link	false	/test e2e-agent-ha-dualstack

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[mkowalski]

One detail I would like to see is a failure of the script if you set EXTERNAL_LOADBALANCER but do not set ENABLE_BOOTSTRAP_STATIC_IP=true. Otherwise we will proceed with openshift-install which will fail and you will end up in a debugging nightmare