Display message from `redis-cli` invocations instead of OK/FAILED
Use present tense instead of present participle
Check for the redis-cli executable if needed

gitlab-shell will use `env -i redis-cli` only if the entire redis
section is missing. If the redis section is present, it will attempt to
use the 'bin' entry in the config.

In the situation where the 'bin' entry is missing and the redis section
is used, system won't print anything about the failure (although it will
fail). This checks for the presence of the 'bin' entry only in
situations where it matters.