Added sentinel exporter support

[Nick-Andreano]

prometheus_exporters.yml's "Install Redis exporter" play already targeted redis_sentinel hosts via its hosts: redis* pattern, but pointed the exporter at the default Redis port (6379) — which doesn't exist on a Sentinel-only host, since Sentinel listens on 26379. The exporter would fail to connect there instead of exposing Sentinel metrics.

This sets redis_exporter_addr to redis://localhost:26379 when the host is in the redis_sentinel group (otherwise unchanged at 6379). The existing oliver006/redis_exporter binary detects Sentinel mode automatically and exposes redis_sentinel_* metrics once pointed at the right port — no new exporter binary or role needed. No changes were required to scrape_configs.j2; its ^redis_.* group loop already includes redis_sentinel hosts in the redis_exporter scrape job.

Also fixed a stale reference in the README to redis_prometheus_user_enabled, renamed to redis_monitor_user_enabled in deployer v4.

Tested against a live 3-node EC2 lab (1 redis_master, 1 redis_sentinel, 1 prometheus) — confirmed Prometheus successfully scrapes redis_sentinel_masters, redis_sentinel_master_status, etc. from the sentinel host, while the master host's exporter continues to expose standard Redis metrics only.