hs-test: nginx mirroring test improvements

- avoid setting LD_PRELOAD for container
- save nginx error log to shared volume
- reduce test run time to 10s
- add vcl and ldp debug env variables to docker file. Default to
disabled.

Type: test

Signed-off-by: Florin Coras <fcoras@cisco.com>
Change-Id: I401ac74e7c0ebe87befedb44150b04f773f244ea

extras/hs-test/resources/nginx/nginx_proxy_mirroring.conf

@@ -1,8 +1,10 @@
-user root;
+master_process on;
 worker_processes 4;
 worker_rlimit_nofile 102400;
 daemon off;
 
+error_log /tmp/nginx/error.log;
+
 events {
   use epoll;
   worker_connections 102400;