BLACKLIST_FILE is used to filter know failing tests from the CI build
and test, and at this time, such feature is only necessary for ppc64le
builds.

Travis don't have a simple way to define per arch "global" variables,
so we set it in build-travis.sh.

To consolidate support for the BLACKLIST_FILE, this variable needs to
be passed along in a couple of places that do not automatically export
the current environment, on the docker-run call and on the build
package stage.

Signed-off-by: Raoni Fassina Firmino <raoni@linux.ibm.com>