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
Fix a bug in ShellFormatter's escaping of double quotes in strings.

Fixes bug: 1302066
Change-Id: I7fe066165ce11cfe977a18c0e49a55ff84ba4187
Merge "Import run_cross_tests.sh from oslo-incubator"