baremetal: enable and document "pod_cidr" and "service_cidr" options

[akurz]

closes #1471

Allow the customization of pod and service CIDRs also for baremetal setups

This is a trivial change to the baremetal platform package that allows a user to define custom values for pod_cidr and service_cidr. Thanks to @invidian for pointing out the necessary code changes :+1:

[akurz]

Anything I can do here to get this PR accepted @invidian?

[invidian]

@surajssd @ipochi @iaguis maybe one of you can have a look?