Only log error when no load handler is set

Only log an error from loading a plugin if no load
failure handler callback is provided. Otherwise,
assume the handler will do any logging it wants.

Change-Id: Iac83f3c140067bc3d1996c40c0184e4a0d0b564a
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"