[18.6] Add nginx['default_server_enabled'] configuration parameter

What does this MR do and why?

This backports !8912 (merged) to 18-6-stable-ee.

In !8805 (merged) we made the GitLab server the default HTTP server because many users were relying on that behavior, which only worked by accident because of the naming of configuration files. However, some users have chosen to define a catch-all in nginx['custom_nginx_config'] to disable all requests that do not use the explicit hostname.

To support this use case, this commit adds a nginx['default_server_enabled'] that can be set to disable the behavior in !8805 (merged).

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The original MR has been deployed to GitLab.com (not applicable for documentation or spec changes).
• This MR has a severity label assigned (if applicable).

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