The board usage will start become long and tedious to maintain.
Let's allow board_families and exclude_board_families for us to get better control of what boards should the task running on.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/infra/suite_scheduler/+/dd833075c552a0fa449b421ee0b9a30dda916925

commit dd833075c552a0fa449b421ee0b9a30dda916925
Author: Po-Hsien Wang <pwang@chromium.org>
Date: Thu Aug 23 20:21:05 2018

suite_scheduler: add exclude_board_families

This CL
1) Adds exclude_board_families to let the user exclude the whole board
families.
2) Change original board_family to board_families, thus allow user to
specify multiple board_family at once.
3) Change the name of the variable 'board_families' aquiared to
board_family_config to avoid confusion.

BUG= chromium:848497 
TEST=python runner.py

Change-Id: I2d86e24a77fa0f32571644ee0c1efc000839916e
Reviewed-on: https://chromium-review.googlesource.com/1179313
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Pohsien Wang <pwang@chromium.org>
Reviewed-by: Xixuan Wu <xixuan@chromium.org>

[modify] https://crrev.com/dd833075c552a0fa449b421ee0b9a30dda916925/config_reader_unittest.py
[modify] https://crrev.com/dd833075c552a0fa449b421ee0b9a30dda916925/config_reader.py
[modify] https://crrev.com/dd833075c552a0fa449b421ee0b9a30dda916925/trigger_receiver_unittest.py
[modify] https://crrev.com/dd833075c552a0fa449b421ee0b9a30dda916925/configs/suite_scheduler.ini
[modify] https://crrev.com/dd833075c552a0fa449b421ee0b9a30dda916925/task.py