[proxy] Migrate current HelmCRD functionality to a REST API

[andresmgot]

As the result of the discussion in https://docs.google.com/document/d/1FPqs5o7vDjKi5icB9XStmXQLeBViljn89ovhtorR3cA we decided to move the HelmCRD functionality from a CRD controller to a REST API to avoid having applications state stored in two different places.

The goal of this issue is to have a REST API working as a proxy to Tiller that allows to create/update/delete releases from public and private registries.