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"
Add doc requirements to venv environ

The venv environ is used to build the documentation, so it
needs to install the doc requirements.

Change-Id: Ice1d5f4e566eade52058dfb7db015aee9cc39909
driver: raise by default on import failure

When using the DriverManager class, if the driver fails to load, it's
actually better by default to re-raise the exception. It's not something
possible when loading multiple extension, but it's safe to do so with
drivers. This just changes the default behaviour, and it can still be
overridden.

The upside of that change is that when you try to load a driver that
cannot be loaded because of missing dependency, you actually get the
ImportError backtrace on your screen rather than the useless:
  RuntimeError: No 'foo' driver found, looking for 'bar'

which doesn't help at all. And the default mechanism that logs via
LOG.error() doesn't print anything at the screen if the application
didn't configure the logging subsystem.

Change-Id: I67d9e13a07c822c54dd16ac9ae7716747a24dd73
Updated from global requirements

Change-Id: I0ebc484c56dc1578246240ac3499bceda0d871a2
Merge "Add doc requirements to venv environ"
Merge "Updated from global requirements"