Speed up Pages initialization time by using configuration API

Description

Once the Pages configuration API is in place it will be necessary to update gitlab-pages to load its configuration from the API instead of the filesystem. This should be done in a way that will allow support for both implementations (API and config file) for a period of time. Shipping two versions of the binary with Omnibus has been considered as an approach.

This will bring the time of pages configuration update from ~30 minutes to almost instant in gitlab.com, including:

1. time for the project to be visible for the first time
2. adding new domains
3. adding SSL certificates

• Add configurable timeout

Proposal

This work has already started in this MR, finishing up that work will complete this issue.

Links / references

Related Epic: &1316 (closed)