REDIS_PASSWORD[_FILE] env and empy password warning

[Peter-Sh]

Experimental proof of concept support for setting password via env

Description

This is proof of concept implementation of env vars for setting password and showing a warning if empty password is detected.

• Provide an ability to set password using REDIS_PASSWORD or REDIS_PASSWORD_FILE variables.
  While it's possible to set password using command line args or configuration files, setting it using env vars is a common and well-known pattern.
• Use --include with temporary config snippet if possible to avoid displaying password in process list
  Fallback to --requirepass if tmp is not writeable or when starting sentinel (it doesn't support include arg)
• Try to detect whether password is already set (use some "heuristic" to detect whether there are already some security related configuration) and show a big warning banner about empty password (which no one would notice as redis and modules output a lot of startup info into terminal)

Open questions

• Allowing to set only password an no other config parameters via env is a bit misleading
• Make redis-cli (and other clients) to use passwords from env or file
• Everything that is marked TODO in debian/docker-entrypoint.sh (a lot of important stuff and edge cases)
• Update documentation describing env vars and ways to set password for older versions (using --include)
• Banner is scrolled out of the screen because a lot of startup logs from redis and modules
• Set password env variables priority or fail when both are set
• Add tests

Related issues

#355
#46
#268