Close #178 Nginx conf default_host documentation

This closes #178
We're just making it clear that some nginx installs such as by default on recent Ubuntu's, the /etc/nginx/sites-enabled/default file will conflict the listen line of the gitlab nginx conf's due to the default_server directive.

changed installation.md to identify the issue to a user
added notes to both nginx configs for gitlab and gitlab-ssl
[ci-skip

See merge request !225