[autotest] Pull suite sharding factor from global_config.ini

Instead of hard-coding a sharding factor in each suite control file,
put a default one in global_config.ini.  Allow overrides.

BUG=chromium-os:26431
TEST=unit
TEST=use dev_harness to run control.bvt, see that it schedules 5 reimaging jobs.
STATUS=Fixed

Change-Id: Ife5a1d117701785530b040ab9b08796b06d57f76
Reviewed-on: https://gerrit.chromium.org/gerrit/15975
Tested-by: Chris Masone <cmasone@chromium.org>
Reviewed-by: Chris Sosa <sosa@chromium.org>
Commit-Ready: Chris Masone <cmasone@chromium.org>
5 files changed