update openstack provider resources to v1beta1

[rapour]

Changes the openstack provider API to v1beta1

• The identityRef.Kind field has been removed. It was used to specify the kind of the identity provider to use but was actually ignored. The referenced resource must always be a Secret.
• The cloudName field has been removed from both OpenStackMachine and OpenStackCluster and added to identityRef. It is now a required field when identityRef is specified.
• In v1beta1, OpenStackCluster.Spec.ManagedSubnets array field is introduced. The NodeCIDR and DNSNameservers fields of OpenStackCluster.Spec are moved into that structure (renaming NodeCIDR to CIDR)
• The field externalNetworkID has been renamed to externalNetwork and is now a NetworkParam object rather than a string ID.
• It is now possible for a user to specify that no external network should be used by setting DisableExternalNetwork to true

[berkayoz]

LGTM overall, have we performed a test deployment with the template?

[rapour]

Yes, but I removed the new disabled external network option to avoid conditional rendering of the template. The clusterctl generate cluster doesn't support Go style templates.