[__nginx_vhost]: follow Alpine vhost default directory change.

Since nginx package version v1.10.1-r3, Alpine packagers have changed
the default vhost directory from conf.d to http.d [0]. This reflects
this change.

[0]: alpine package commit 383ba9c0a200ed1f4b11d7db74207526ad90bbe3

type/__nginx_vhost/manifest

@@ -32,7 +32,7 @@ case "$os" in
 
 		require="$install_reqs" __start_on_boot nginx
 
-		export NGINX_SITEDIR="$nginx_confdir/conf.d"
+		export NGINX_SITEDIR="$nginx_confdir/http.d"
 		export NGINX_CERTDIR="$nginx_confdir/ssl"
 		export NGINX_SNIPPETSDIR="$nginx_confdir/snippets"
 		export NGINX_WEBROOT="/var/www"
@@ -158,6 +158,7 @@ for snippet in hsts 301-to-https; do
 done
 
 # Install vhost.
-require="$install_reqs" __file "$NGINX_SITEDIR/$__object_id.conf" \
+require="$install_reqs" __directory "$NGINX_SITEDIR"
+require="__directory/$NGINX_SITEDIR" __file "$NGINX_SITEDIR/$__object_id.conf" \
 	--source "$vhost_conf" \
 	--mode 0644