This commit adds support for using redis-exporter with an
external Redis instance when enabled via:

```
redis_exporter['enable'] = true
```

Omnibus GitLab disables redis-exporter if the bundled Redis instance
is disabled, but previously when redis-exporter were enabled it
would always attempt to talk to a local UNIX socket.

This commit has been tested with Google Memorystore with TLS enabled.
Note that TLS client auth is not yet supported.