Explicitly support `SentinelPassword` in workhorse's config.toml for go-redis

The following discussion from !128204 (merged) should be addressed:

• @stanhu started a discussion: (+7 comments)

  FYI, we're going to want to support SentinelPassword as well in the future.

Presently, gitlab.com does not use requirepass for sentinels (only the Redis instances). But with support added in #235938 (closed) + go-redis explicitly require SentinelPassword, it would be clearer to explicitly support it via config.toml.

The Redis sentinel password is supported in omnibus-gitlab at the moment (omnibus-gitlab!6921 (merged)) but config.toml only loads 1 password for the Redis instance (it assumes sentinel passwords are in the urls). This would mean that we cannot configure Redis and Sentinels with different passwords (see !128204 (comment 1522217171)).

Possible tasks (I'd imagine this needs to go into the same release)

1. Add SentinelPassword usage into config.toml to support sentinel passwords + release go-redis client (alongside deprecating redigo client)
2. Update omnibus-gitlab and charts to render the appropriate config.toml