Check for and export success status in downstream

This commit updates the downstream job to check for the success of the repo job
(which will have failed if either package build or repo creation have failed).
Depending on the result, the downstream job exports
PACKAGE_BUILD_STATUS="success" or "failure." This variable will then be
appended as a parameter to whatever was supplied as the downstream url, so that
the job can be notified.

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>