Add solution for when instance log output is empty

A Linux instance has to be configured to send log output to the serial
console whenever the Compute service collects log output from there.
Otherwise, the log output will be empty. Added this as a
troubleshooting tip to Cloud Admin Guide.

Change-Id: I5adb28c2d73c41792c1d67f51e11edfc7474ed99
Closes-Bug: #1284962
Merge "Add solution for when instance log output is empty"