machine-api-provider-openstack imports cluster-api-provider-openstack as a library.

Before this change, MAPO's go.mod points to the upstream CAPO: sigs.k8s.io/cluster-api-provider-openstack

After this change, MAPO's go.mod points to the downstream CAPO: github.com/openshift/cluster-api-provider-openstack

As a prerequisite, downstream CAPO's repository must contain a branch mirroring upstream CAPO.

CAPO upgrades will be carried out manually for the time being.