PiEdge deployment instructions with OpenVPN

[amalbet]

User Story
As a technician on site, I would like to have instructions on how to setup a Pi Edge device so I can deploy it onsite and integrate it with my meters.

Acceptance Criteria

• A documentation and installation script is available
• Procedures installs:
  -- the OpenEMS Edge components
  -- OpenVPN client
  -- Any required configuration
• The deployed Pi Edge device is able to connect to the technician's device with local OpenVPN (see Technician is able to connect laptop to a Pi Edge over VPN #27)