sync_package_status: Support disabling issue creation per package.
sync_package_status: Support disabling issue creation per package.

With this change, a package can use the value "disable" in the Tracker
column of the packages spreadsheet to disable the automatic filing of
new Tracker issues for that package.  This usage may be more clear than
leaving a tracker issue (generally one marked WontFix).  It also has the
benefit of being sticky even if the package is upgraded (which typically
clears the Tracker value for the package).

BUG=chromium:325007
TEST=unittest
TEST=~sync_package_status --team=build --pretend` repeated with various
values for glibc package: "", old issue, "disable".

Change-Id: I293db41fa252dcd6b235537efefe959bb493237a
Reviewed-on: https://chromium-review.googlesource.com/178545
Tested-by: Matt Tennant <46e612415b446ccde19638e4035892001c237942@chromium.org>
Reviewed-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Commit-Queue: Matt Tennant <46e612415b446ccde19638e4035892001c237942@chromium.org>

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