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))