Custom Mattermost nginx configuration params not picked up

Summary

Custom Mattermost nginx configuration params not picked up.

According to gitlab.rb any parameters for nginx will work for mattermost_nginx.

This was originally reported in: #4353 (closed)

However only referrer policy was fixed.

Steps to reproduce

Add following lines to gitlab.rb

mattermost_nginx['hsts_max_age'] = 31536000
mattermost_nginx['hsts_include_subdomains'] = false

What is the current bug behavior?

Mattermost Nginx configuration is not written in gitlab-mattermost-http.conf

What is the expected correct behavior?

Following code should be in gitlab-mattermost-http.conf

## HSTS Config 
## https://www.nginx.com/blog/http-strict-transport-security-hsts-and-nginx/
add_header Strict-Transport-Security "max-age=31536000";

Details of package version

Provide the package version installation details

gitlab-ce           12.10.6-ce.0

Configuration details

Provide the relevant sections of `/etc/gitlab/gitlab.rb`

mattermost_nginx['hsts_max_age'] = 31536000
mattermost_nginx['hsts_include_subdomains'] = false