Import run_cross_tests.sh from oslo-incubator

Add the script for running unit tests in other
projects so we can more easily establish
cross-project gate jobs.

Depends on https://review.openstack.org/#/c/83411/
and https://review.openstack.org/#/c/83412/

Change-Id: I0c4b1c425c3aea18b7825c75b9147fd8a1da6914
Merge "Import run_cross_tests.sh from oslo-incubator"