Feat/no download source

The PR allows the redis source to be copied over ssh rather than being downloaded. This is useful for machines in restricted environments or in i.e dmz networks. If `redis_download` is set to `false` `files/redis-{{ redis_version }}.tar.gz` is copied over ssh.
Change keeps the download from redis.io as default behaviour.
@slapers Could you add a commit to fix the spacing here? This should read `{{ redis_tarball }}`. Should be good to merge after that.