Move gitaly enabled flag to global

Summary

https://gitlab.com/charts/gitlab/issues/840#note_116635378

There should be no need to set global.gitaly.internal=[] in additional to gitlab.gitaly.enabled=false. We should move this to global.gitaly.enabled and gate the gitlab.gitaly.storage.internal template based on that. There should not be the possibility of having internal services populated when they are not operational.

Current behavior

Currently, it is required to set one of the following to properly disable internal gitaly but neither is clearly documented:

global:
  gitaly:
    internal: []
    external:
      - name: x
        hostname: x.example.com
      - name: y
        hostname: y.example.com
gitlab:
  gitaly:
    enabled: false

global:
  gitaly:
    host: only.example.com
gitlab:
  gitaly:
    enabled: false

Expected behavior

The following configuration should function as expected.

global:
  gitaly:
    enabled: false
    external:
      - name: x
        hostname: x.example.com
      - name: y
        hostname: y.example.com

cc @gitlab-org/distribution