redis: Ignore snapshots dumps

What does this Merge Request do and why?

Recently redis started to create a dump.rdb when it is stopped. You can see this happen when you run gdk start redis and gdk stop redis. This dump shouldn't make the GDK repository dirty.

This change adds the file to .gitignore. And because the dump belongs to redis, the configuration file is changed to save that file in the redis/ subdirectory of GDK.

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.