Release 2.3.1: fix bug with RPi 3

[dputtick]

• The way network interfaces are named changed from the RPi 2 to 3. The name in ifconfig now includes the mac address of that individual RPi. When rc.local tried to turn off eth0, the ifconfig call was failing since the device didn't exist, preventing rc.local from finishing and the groomer from running.
• This PR fixes #60 by removing that call, and instead turning off networking.service in the prebuilt image config. This is a (probably) more elegant way to accomplish the same goal.
• I also turned off a few other services that we didn't need (bluetooth.service and dhcpcd.service), which makes the RPI boot dramatically faster.
• Also includes some documentation updates.
• This was a pretty unfortunate bug, and we definitely need to be more systematic and careful about testing releases. I'm going to write some release acceptance criteria explicitly spelling out how release testing should be carried out to prevent this from happening again!

[Rafiot]

Excellent, thank you very much!