[CA lifecycle] Allow server-url override on MachineRegistration

[anmazzotti]

When generating the MachineRegistration's url, the elemental-operator reads the server-url Rancher setting value.

It should be possible for users to override this value, so that it would be possible to route different registrations to different load balancers.
In combination with #859 and #858, this could also allow using different CAs per-registration and have strict validation, for example in a multi-tenant environment.