Redis Sentinel daemon

Related to #1565 (closed) !996 (closed)

Updated documentation:

• CE: gitlab-org/gitlab-ce!6471
• EE: gitlab-org/gitlab-ee!786 (see: https://gitlab.com/gitlab-org/omnibus-gitlab/merge_requests/1000#note_17084207)

Checklist

• Create a template for sentinel.conf
  • port is configurable
  • working-directory is configurable
  • auth-pass is configurable
  • monitor master instance is configurable
  • down-after-milliseconds is configurable
  • failover-timeout is configurable
  • myid is configurable or will fallback to generated and stored in .json file after first reconfigure
• Create a service definition for sentinel daemon (definitions/sentinel_service.rb)
• Sentinel code is EE package only (gitlab-ee recipe)
• Add sentinel to runit init system
• Sentinel should be enabled only when specific enabled flag is present
• Sentinel-disable recipe
• Generate specific redis:// URL pointing to master-name when sentinels are present

Testplan

• Clean install:
  • Works with default / unmodified configuration parameters
  • Can enable managed sentinel
  • Can disable managed sentinel (stop service and remove from runit)
  • Can point to external (managed/unmanaged) sentinel service
  • Works with multiple sentinel instances.
• Existing previous version
  • Can upgrade with default / unmodified configuration parameters
  • Can enable managed sentinel
  • Can disable managed sentinel (stop service and remove from runit)