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/script/nginx_ldp.sh

@@ -0,0 +1,3 @@
+#!/usr/bin/env bash
+
+LD_PRELOAD=$LDP nginx $@ 2>&1 > /proc/1/fd/1