Support for Redis connection without SSL certificate verification

[tomash]

Context: on Heroku Key-Value addon (Valkey fork of Redis), they use TLS but with a self-signed certificate, as described here:

• https://devcenter.heroku.com/articles/connecting-heroku-redis#external-connections
• https://devcenter.heroku.com/articles/heroku-redis#security-and-compliance

This PR adds REDIS_SSL_VERIFY environment variable to make it work without patching Pubsubstub's class in-application.

A similar PR is coming to shipit-engine. EDIT: Shopify/shipit-engine#1397

No test coverage because no existing spec covers Redis connection, should I add some?

[tomash]

I have signed the CLA!

[byroot]

There is a million option like this you may need. Any reason why the redis= method doesn't work for you?