cbuildbot_commands.py: call sync_package_status in refresh-packages builder.

Adding calls to sync_package_status at the end of the refresh-packages stage.
This will automatically synchronize tracker issues for outdated packages in
the package spreadsheet, but only for groups that have opted in.  Adding more
groups is simply a matter of making a minor change to cbuildbot_commands.py.

Refactored a couple locations that had hard-coded paths to "chromite/bin" to
use new entry in constants.py.

BUG=chromium-os:25674
TEST=`sync_package_status --pretend --owner=mtennant` behaves normally
TEST=pylint of all files
TEST=`python sync_package_status_unittest.py` passes
TEST=trybot of refresh-packages passes and log shows results of
  sync_package_status run with --pretend.

Change-Id: Iebc8242e7ef4fff5b61afc8232a464dd291d0146
Reviewed-on: https://gerrit.chromium.org/gerrit/17279
Tested-by: Matt Tennant <mtennant@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Ready: Matt Tennant <mtennant@chromium.org>
diff --git a/scripts/cbuildbot.py b/scripts/cbuildbot.py
index feb472e..7907ef3 100644
--- a/scripts/cbuildbot.py
+++ b/scripts/cbuildbot.py
@@ -44,7 +44,7 @@
 _BUILDBOT_LOG_FILE = 'cbuildbot.log'
 _DEFAULT_EXT_BUILDROOT = 'trybot'
 _DEFAULT_INT_BUILDROOT = 'trybot-internal'
-_PATH_TO_CBUILDBOT = 'chromite/bin/cbuildbot'
+_PATH_TO_CBUILDBOT = os.path.join(constants.CHROMITE_BIN_SUBDIR, 'cbuildbot')
 _DISTRIBUTED_TYPES = [constants.COMMIT_QUEUE_TYPE, constants.PFQ_TYPE,
                       constants.CANARY_TYPE, constants.CHROME_PFQ_TYPE,
                       constants.PALADIN_TYPE]