Workload cluster CAPM3 network refactoring

Issue created in the context of epic &45 (closed).

Workload cluster definition requirements for metal3 networks:

• ability to create additional networks besides the primary network;
• deprecate provisioning network definition (not needed in DHCP-less context);
• ability add static routes towards next-hops from additional networks; Caution: These static routes need to be quite specific, since having summary/aggregate routes like 10.0.0.0/8 could break the reachability for critical cluster services. So a careful analysis of the static routes that are added on additional networks needs to be done during network cluster design phase and is out of the scope for this issue.

Note: ability to change the default gateway from the primary network to an additional network is also a good improvement for the CAPM3 workload cluster definition, but it is not a supported setup on workload clusters, since it could break cluster reachability.