gen_nginx_config.sh: change filename semantics of rules files

if sites.txt
	sites.txt -> plain-only
elseif https.txt
	http.txt -> plain-only
	https.txt -> ssl-only
else
	http.txt -> plain and ssl
	http-only.txt -> plain-only
	https-only.txt -> ssl-only

to reduce duplication in the generated config

Signed-off-by: Alejandro Mery <amery@geeks.cl>