Management Controller should handle a CertificateRequest resource for sk2

[fgiorgetti]

During the backbone Bootstrap step 2, users need to run skxhosts to get a list of hosts that will be used by the Management Controller to sign the Server Certificate.

We need to change this, and request user to provide the content of the CertficateRequest resource on step 2.

The Management Controller will then, process the CertificateRequest and update it with the public server certificate and public CA certificates.

Then the Bootstrap step 3 which returns a finish yaml, would just return an updated version of the CertificateRequest to be applied at the backbone site's namespace.