Allow empty pools

This pull request includes an update to allow a range to be omitted. This is useful for use-cases where the dhcp::host section is used primarily and there should not be any DHCP addresses offered unless it matches a particular host.

    dhcp::pool{ 'network':
                network => '10.1.168.0',
                mask    => '255.255.255.0',
                gateway => '10.1.168.1',
    }

    dhcp::host {
          'server1': mac => "00:50:56:b7:60:d5", ip => "10.1.168.206";
    }