Fix security vulnerabilities in suggested nginx configuration

The suggested configurations for nginx found in the documentation and
templates lead to vulnerabilities allowing host spoofing [1] and path
traversal [2], as reported by Gixy [3]. This commit fixes those issues.

[1] https://github.com/yandex/gixy/blob/master/docs/en/plugins/hostspoofing.md
[2] https://github.com/yandex/gixy/blob/master/docs/en/plugins/aliastraversal.md
[3] https://github.com/yandex/gixy