mod_proxy_html failing in Debian

proxy_html needs to load libxml2 library before loading module in Debian
Allow passing list of files to load before loading the module to apache::mod
libxml2 path in i686 hardwaremodel is /usr/lib/i386-linux-gnu/libxml2.so.2
Add acceptance tests for proxy_html and debian 7.3

Tested with beaker in debian-73-i386, debian-73-x64, centos-65-x64, ubuntu-server-1310-x64 (with patch in Issue #672)