rebaseline_server: handle column filtering more generically

This is a step towards making rebaseline_server more flexible, and useful for displaying output of the cluster telemetry server as well as actuals-on-configA-vs-actuals-on-configB.

BUG=skia:2230
R=rmistry@google.com

Author: epoger@google.com

Review URL: https://codereview.chromium.org/369133004
3 files changed