Add openvpn monitor support

[m0wer]

It would be nice and it's easy to implement.

This configuration works:

docker run --name openvpn-monitor \
  -e OPENVPNMONITOR_DEFAULT_DATETIMEFORMAT="%%d/%%m/%%Y" \
  -e OPENVPNMONITOR_DEFAULT_LATITUDE=-44 \
  -e OPENVPNMONITOR_DEFAULT_LOGO=logo.jpg \
  -e OPENVPNMONITOR_DEFAULT_LONGITUDE=-137 \
  -e OPENVPNMONITOR_DEFAULT_MAPS=True \
  -e OPENVPNMONITOR_DEFAULT_SITE=Test \
  -e OPENVPNMONITOR_SITES_0_ALIAS=UDP \
  -e OPENVPNMONITOR_SITES_0_HOST=openvpn \
  -e OPENVPNMONITOR_SITES_0_NAME=UDP \
  -e OPENVPNMONITOR_SITES_0_PORT=5555 \
  --link openvpn \
  -p 8123:80 ruimarinho/openvpn-monitor