Move SSL settings to separate config files

[cristoper]

For each entry in nginx_sites, create a file at /etc/nginx/conf.d/common_ssl_site.<domain>.conf containing its SSL configs, and include that in its main configuration files. This should fix upstream Issue #15.

Also create /etc/nginx.conf.d/common_ssl_default.conf with the default SSL settings. This is convenient for other vhosts -- including those not managed by this role -- to be able to import.