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
Add a CentOS 6.5 beaker node
Merge pull request #673 from maestrodev/proxy_html

mod_proxy_html failing in Debian