Add udp and ports specification for openvpn.sh

Hello,                                                                                                                                                         
                                                                                                                                                               
I need to use check_vpn with an openvpn server in udp and a specific port. So I made some modification on check_vpn_plugins/openvpn.sh                         
                                                                                                                                                               
 * no nmap check if udp (the port seem already open, even if its close, nmap -Pn -sU -p 20000 google.com | grep -q "20000/udp open" will be open true)         
 * --connect-retry is a tcp only args, so I moved it on a protocol_extra_args vars.                                                                            
                                                                                                                                                               
 To use this, you need to exec :                                                                                                                               
                                                                                                                                                               
 OPENVPN_PORT=443 OPENVPN_PROTOCOL=udp ./check_vpn -t openvpn -H  you_server.com -u user -p password -- extra_args                                             
                                                                                                                                                               
 Hope this script will be helpfull for another people                                                                                                          
                                                                                                                                                               
 Nico