[do not merge] Default Kubernetes routes to http when the scheme is missing

[guicassolato]

Rolls back https://github.com/3scale/zync/pull/321 and relies on the new API attributes base_url and admin_base_url just introduced in the 3scale Admin API.

:warning: Attention! Do not merge it until https://github.com/3scale/porta/pull/1869 is merged and deployed, otherwise it will just roll back https://github.com/3scale/zync/pull/321 and reopen THREESCALE-4832.