VPN: support ZeroTier by default

[rmackay9]

I think ZeroTier is so commonly used that it would be good to have it included by default in the RPanion Server image instead of users having to login to the RPI and install it manually.

BTW, to install ZeroTier on RPanion Server do this:

1. ensure the RPI has access to the internet (e.g. install on the RPI I/O board with a physical Ethernet connection)
2. power on the RPI
3. connect to the rpanion wifi AP
4. use Putty (or similar) to SSH to the Rpanion (IP: 10.0.2.100, Port:22, Connection Type:SSH, login: pi, pw:raspberry3)
5. follow the Linux instructions here which is just, "curl -s https://install.zerotier.com | sudo bash"

The VPN section of the RPanion Server v0.12 user manual can be found here

[stephendade]

Now included

[rmackay9]

Thanks so much!