extract installer binary, instead OVERRIDE and use pre-configured network.

[sallyom]

This PR:

1. Updates create-cluster to extract the libvirt installer from the given RELEASE_IMAGE, rather than set OPENSHIFT_INSTALL_RELEASE_IMAGE_OVERRIDE. Temporarily handle both variables to transition CI and not break libvirt test (setup should extract installer rather than build it.  #32).
2. Uses a pre-configured, shared network & subnet with firewall rules set for accessing the gcp instance. This is ocp4-libvirt-dev in the openshift-gce-devel gcp project.
3. Adds a create-network-subnet.sh script in case one does not want to use the shared network.
4. Modifies the create and teardown scripts to remove the network, subnet, firewall creation.
5. Downgrade the qemu-kvm version in gcp image (qemu-kvm version issue in rhel 8.2 gcp image #29)

[sallyom]

closing, in favor of this: #34