Add SERVICE_TOKEN to the sample local.conf

I expect the sample local.conf file to contain all the required
variables. However, after copying samples/local.conf, the stack.sh
script still asks for SERVICE_TOKEN.

This commit adds the SERVICE_TOKEN definition to local.conf so that a
new user can just copy samples/local.conf, edit it, and have a running
stack.sh without the mild annoyance of having more variables to set.

Change-Id: Ifb163c7f67c80037d8b10458f2df458a20da4667
Merge "Add SERVICE_TOKEN to the sample local.conf"