add option in vhost class to set ProxyPreserveHost in virtual host config
add spec test for the new ProxyPreserveHost option
add "template uses" comment
add short explanation about proxy_preserve_host option in readme
Merge branch 'ProxyPreserveHost' of https://github.com/mohamedhadrouj-vv/puppetlabs-apache into mohamedhadrouj-vv-ProxyPreserveHost

Conflicts:
	templates/vhost/_proxy.erb