Add gitlab.rails.application_settings_cache_seconds config variable

What does this merge request do and why?

Adds config for GitLab's application settings cache, like the config for omnibus-gitlab. It's needed in e2e tests to disable the cache to prevent test flakiness when settings (including feature flags) are toggled on and off.

See also gitlab#217010 (closed)

How to set up and validate locally

1. Adding the below configuration in gdk.yml:

   gitlab:
     rails:
        application_settings_cache_seconds: 0

2. Check out to this merge request's branch.
3. Run gdk reconfigure to check if regenerating all configuration is successful.

The change should be similar to:

'gitlab/config/gitlab.yml' has incoming changes:
-------------------------------------------------------------------------------------------------------------
@@ -108,6 +108,9 @@ production: &base
     ## Impersonation settings
     impersonation_enabled: true

+    ## Application settings cache expiry in seconds (default: 60)
+    application_settings_cache_seconds: 0
+
   ## Reply by email
   # Allow users to comment on issues and merge requests by replying to notification emails.
   # For documentation on how to set this up, see http://doc.gitlab.com/ce/administration/reply_by_email.html

-------------------------------------------------------------------------------------------------------------

Compare with gitlab/config/gitlab.yml.example.

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.