gather_builder_stats.py: Add an --override-ss-key flag
gather_builder_stats.py: Add an --override-ss-key flag

This CL refactors the way the spreadsheet key is stored (it now belongs
to the StatsManager that is doing the spreadsheet writing, which makes
more sense than the Table class that describes the data being stored).
This makes it possible to override the spreadsheet for testing purposes.

A --no-sheets-version-filter flag is also added for testing purposes.

BUG=chromium:367869
TEST=Ran through the usual gather_builder_stats with overridden
spreadsheet and --no-mark-gathered --no-carbon

Change-Id: Iba4fadb16298fe566e7ae562a985f7dacf99a4d7
Reviewed-on: https://chromium-review.googlesource.com/197508
Reviewed-by: Aviv Keshet <178e8ae417efbe404feca7ab4cb8f0ba8c109369@chromium.org>
Tested-by: Aviv Keshet <178e8ae417efbe404feca7ab4cb8f0ba8c109369@chromium.org>

bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite