The `Rails` object was not always available in all tasks that require
Redis access, such as `mail_room`, so the constant pointing to the
configuration path was never defined, but we still attempted to access
it in `config_file_name`, resulting in a `NameError` exception.
Further, there was no benefit to defining these paths in a constant to
begin with -- they're only accessed in one place, and it was within the
class where they were being defined. We can just provide them at
run-time instead.
Further _still_, we were calling `File.expand_path` on the absolute path
returned by `Rails.root.join`, which was rather pointless.
These two methods now handle two additional real-world possibilities:
1. `config/resque.yml` not being present
2. `config/resque.yml` being present but not containing YAML
Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/34941
