Allow to read /etc/redis/redis.conf

[dualmi]

Makes readable /etc/redis/redis.conf and allows to configure redis snap with it.

Additional systemd modifications requred:

1. Run systemctl edit snap.redis.server
2. Add following code to provide config path to redis executable (empty ExecStart required)

[Service]
ExecStart=
ExecStart=/usr/bin/snap run redis.server /etc/redis/redis.conf

[yossigo]

@keir-ru Thanks for this PR, I have a few comments:

• Perhaps it will be a better idea to make all of /etc/redis readable? Depending on deployment, there could be other files as well.
• While at it, what about writable configuration files? For example, acl file, cluster nodes.conf, sentinel.conf or even redis.conf when CONFIG REWRITE is used.
• I think it would be a good idea to update the README.md to add a section on running the snap package with systemd and include a more detailed example.