Use --cap-add=NET_ADMIN instead of --privileged

Ovpn doesn't need all the capabilities.
https://docs.docker.com/reference/run/#runtime-privilege-linux-capabilities-and-lxc-configuration says:
For interacting with the network stack, instead of using --privileged they should use --cap-add=NET_ADMIN to modify the network interfaces.
Merge pull request #10 from adrianolek/patch-1

Use --cap-add=NET_ADMIN instead of --privileged