Add Backend functionality and ACL support

The follow class defines backends with a supporting template.
The ACL template is added to the listen clause to allow routing to the correct backends

eg.

    @@haproxy::backend { "$application":
        listening_service => 'HTTP-Frontend',
        mode              => 'http',
        server_names      => $::hostname,
        ipaddresses       => $::ipaddress,
        ports             => $application_port,
        check             => 'true',
        options           => {
            'option'      => 'httpchk GET /check HTTP/1.0',
            'balance' => 'roundrobin'
        },
        acls              => {
           'acl' =>  "${application} hdr_beg(host) -i www",
           'use_backend' => "${application} if ${application}"
        }
    }