Add omnibus chef to renovate

This MR migrates chef-related dependencies of omnibus-gitlab from deps.io to Renovate.

One problem with deps.io is the Rubygems datasource updating before packages were uploaded. This sync latency caused problems when we used the canonical chef repo for versioning information and cinc packagecloud for the actual packages.

To solve this in renovate, we were able to introduce a separate bundler manager in addition to our regex manager. This bundler manager allows us to gracefully handle querying packagecloud for both the versioning information as well as package downloads.

We did not yet include mixlib-log because it currently conflicts with the pinned version from ffi.

Currently, the syntax is verbose and not DRY, which we will be fixing as we iterate and learn more about Renovate syntax.