Fixed: #4307 The new LE ssl cert paths introduced in ISPConfig 3.1.1 where not set in the nginx_vhost.conf.master

The cipher suites are only set now for ISPConfig's own vhost, the intermediate security profile from Mozilla is used here.

- backported patches

see:
- http://git.ispconfig.org/panders/ispconfig3/commit/f7ec00b2f8ba3efc5bdeacef9c813f8a826ae3be#note_454

- http://en.wikipedia.org/wiki/SPDY

- nginx: merge function: default action is now to replace older locations in the nginx vhost configuration; a merge  instead of a replacement can be achieved by putting the string "##merge##" (without quotes) at the end of the most recent location line (e.g. "location ~ \.php$ { ##merge##").

- Added PHP-FPM pm settings (pm.max_children, pm.start_servers, pm.min_spare_servers, pm.max_spare_servers) to the "Options" tab of a website.

- Changed FastCGI buffers for ISPConfig nginx vhost because otherwise you get "upstream sent too big header while reading response header from upstream" on OpenSUSE.
- Added 502 error page ("Bad Gateway").
- Added "fastcgi_intercept_errors on;" to nginx vhost template so that ISPConfig's error pages are displayed instead of the default nginx ones.

- Added "ipv6only=on" to the IPv6 listen lines in the nginx vhost template to avoid "[emerg]: bind() to [::]:80 failed (98: Address already in use)" errors.