Serve /kernelrepo through nginx so minions can reach the kernel repo

The /nsm/kernelrepo bind mount exposed the files, but without a matching
location block external requests to /kernelrepo/ fell through to the SOC
app and returned HTML, so minions hit 'repomd.xml parser error'. Add a
/kernelrepo/ location mirroring /repo/.

salt/nginx/etc/nginx.conf

@@ -323,6 +323,16 @@ http {
 			autoindex_localtime on;
 		}
 
+		location /kernelrepo/ {
+			allow all;
+			sendfile on;
+			sendfile_max_chunk 1m;
+			autoindex on;
+			autoindex_exact_size off;
+			autoindex_format html;
+			autoindex_localtime on;
+		}
+
 		location /influxdb/ {
 			auth_request          /auth/sessions/whoami;
 			rewrite               /influxdb/api/(.*) /api/$1 break;