Re integrate refactored VPN code into Bitmask

Integrate the @leap_vpn@ code currently living on https://github.com/ivanalejandro0/leap_vpn/ as @leap.bitmask.vpn@ on bitmask_client repo.

This continues the work started on #7732 (closed)

(from redmine: created on 2016-02-13, relates #7732 (closed))