Import run_cross_tests.sh from oslo-incubator

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

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

Change-Id: I3d3e25f92ab6337432b54967a381662f32902a6d
Merge "Import run_cross_tests.sh from oslo-incubator"