Support connecting to Redis Sentinel (EE only)

This is supported natively by redis-rb, but it is not supported at this time by our Redis config format (resque.yml).

We need to make sure the right Redis config gets picked up in at least three places: Sidekiq, cache and session store.

Having Redis Sentinel support would allow us to have HA redis on gitlab.com. gitlab-com/operations#5 (closed)

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Sentinel support components:

• Sentinel configurable in resque.yml (!1877 (merged))
• Sidekiq uses Sentinel configuration (!1877 (merged))
• GitLab uses Sentinel for cache / session connection (!1877 (merged))
• GitLab mailroom's (reply by email support) uses sentinel - will be in 8.14
  • #21645 (closed)
  • !7101 (merged)
  • https://github.com/tpitale/mail_room/pull/79
• gitlab-shell supports configuring sentinel in gitlab.yml (gitlab-shell#29 (closed) / gitlab-shell!85 (merged))
• Omnibus allows configuring Sentinel hosts for GitLab (omnibus-gitlab!882 (merged))
• Omnibus allows Redis Replication setup (omnibus-gitlab!882 (merged))
• Omnibus generates gitlab.yml for gitlab-shell with Redis Sentinel support (omnibus-gitlab#1512 (closed) / omnibus-gitlab!948 (merged))
• Omnibus allows Redis Sentinel to be configured and managed - will be in 8.14
  • omnibus-gitlab#1565 (closed)
  • omnibus-gitlab!1000 (merged)
• Redis Sentinel for HA documentation
  • gitlab-org/gitlab-ce!6471
  • gitlab-org/gitlab-ee!786