Redis 5 for CentOS 7

[enbrnz]

This changes the redis installation in CentOS 7 to the SCL package which provides a more up to date version (Redis version 5) DONE:

• created new variables in vars/{debian,redhat}.yml
• removed variables from defaults/main.yml
• adjusted testing
• create handling for new variables in tasks/overwrites.yml
• update templates/redis.conf.j2 to apply current default configuration for Redis 5 (also need to make sure this still works on Ubuntu 18.04)

[enbrnz]

closes issue #4