(#272) Add parameter sslproxyengine to vhost.pp

In order to use mod_proxy with remote servers using the SSL/TLS protocol it is
needed to activate the SSL/TLS Protocol Engine for proxy. Setting this
parameter to true adds the directive `SSLProxyEngine on` to the virtual host.
Merge pull request #273 from mattthias/feature/master/add_sslproxyengine_to_vhost

(#272) Add parameter sslproxyengine to vhost.pp