Fixes issue 415 - set_real_ip_from should support a list

The set_real_ip_from paremeter from the ngx_http_realip_module may be specified
multiple times within the http, server & location contexts. This commit adds
list support whilst preserving original behaviour (single-valued string).
Resolves issue nginxinc#415.

templates/http/modules.j2

@@ -255,7 +255,11 @@ mirror_request_body {{ mirror['request_body'] | ternary('on', 'off') }};
 {# NGINX HTTP RealIP -- ngx_http_realip_module #}
 {% macro realip(realip) %}
 {% if realip['set_real_ip_from'] is defined %}
+{% for set_real_ip_from in realip['set_real_ip_from'] if realip['set_real_ip_from'] is not string %}
+set_real_ip_from {{ set_real_ip_from }};
+{% else %}
 set_real_ip_from {{ realip['set_real_ip_from'] }};
+{% endfor %}
 {% endif %}
 {% if realip['real_ip_header'] is defined %}
 real_ip_header {{ realip['real_ip_header'] }};