Base staging configuration on the production one

[javierm]

References

• We did a similar simplification for the preproduction environment in commit 7b91adb from pull request Simplify staging/preproduction environment files #4875

Objectives

• Make it easier to add changes that affect both the production and staging environment
• Make it easier to upgrade to new versions of Rails

Notes

Back in pull request #4875, we said:

After these changes, the staging and production environments are mostly identical, and we could simply require the production environment configuration in the staging.rb file. We aren't doing so because we're doing changes affecting the preproduction environment and, since [Consul Democracy] installations might have customized these files, it would be hard for them to deal with all those changes at once.

We've released a couple of version Consul Democracy since then, so now we can do the same for the staging environment without existing installations having to deal with too many changes.

Release notes

⚠️ Other than the standard production, development and test environments, Consul Democracy offers the option to setup staging and preproduction environments. If you're using the staging environment, note we're now base the staging configuration on the production one. If your config/environments/staging.rb is not identical to your config/environments/production.rb, you'll have to change the former in order to include the differences (we've already included the difference related to the log_level). If you've customized these environments using the config/environments/custom/staging.rb or config/environments/custom/production.rb files, you might need to change the former to take this change into account. If you aren't using the staging environment, ignore this warning.