Add cell.id as configuration field

Overview

Initially, we will route Private Access Tokens and CI Tokens via Cell ID so each GitLab Installation needs to have a Cell ID attached to it. The value of the Cell ID will be an integer but this is yet to be determined what generates that cell ID.

Inside of Rails, we need to have a configuration field to specify the ID of that instance. This configuration field will need to be made available for omnibus and helm chart for completeness's sake. It is also required for Cell seeding for Cells 1.0 Phase 7 (&15565)

Action

• Introduced new cell.id configuration field. - !171105 (merged)
• Add the ability to set it in GDK, and each Cell should have it's own unique ID - gitlab-development-kit!4139 (merged)
• Add the ability to set it in omnibus
• Add the ability to set it in helm charts.