Add environment variable REDIS_PASSWORD

[mrolli]

This change allows to set a Redis password without the need to build up a whole REDIS_URL environment variable. The change is backward compatible as the default is set to "None", which is the default for redis.StrictRedis() according to the documentation.

With this change, deployment in a container app on Azure using a add-on Redis is then super simple as the env vars REDIS_HOST, REDIS_PORT and REDIS_PASSWORD are injected automatically by the platform.

Fixes #369