Redis: Fix templating of secret when password.enabled=false

Address #2051 (closed) by moving the if condition within gitlab.redis.secret This is accomplished by including gitlab.redis.configMerge, which is safe to re-use because of the gating within that function, having no real impact on execution time.

This also ensure the complication of the if-case stays outside of the caller.

Closes #2051 (closed)