OpenVPN

[rubenhorn]

Connect to a private cluster using OpenVPN

When working with multiple private clusters from different organizations, it would be convenient if FabSim could automatically connect to and disconnect from the respective networks.

I have added a simple class that starts the OpenVPN client and connects with the configuration specified in the environment. After executing the remote commands, the VPN process is killed, which closes the connection.

[djgroen]

This passes all tests and the code changes look sensible without a serious increase in dependency footprint. So I'll merge it in :).