Discovery ignores plugin errors

[Marc Koderer]

In case of an error in a Tempest plugin code the discovery will simply
ignore it. This can be problematic in the gate since it could be possible
that tests are skipped without any notice.

[Marc Koderer (m-koderer)]

Steps to reproduce:

# Defined plugin class with missing function
class ManilaTempestPlugin(plugins.TempestPlugin):
     pass

function "load_tests" ignores plugin even though it's specified in setup.cfg.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/202012

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/202012
Committed: https://git.openstack.org/cgit/openstack/tempest/commit/?id=25319f6fa08c628b310ac6468bdc5db827b1b81b
Submitter: Jenkins
Branch: master

commit 25319f6fa08c628b310ac6468bdc5db827b1b81b
Author: Marc Koderer <email address hidden>
Date: Wed Jul 15 11:28:38 2015 +0200

    Stop test execution in case of an plugin error

    Without setting the on_load_failure_callback parameter
    failures are ignored (only logged). This can be critical in
    gate jobs.

    Change-Id: I820784d759d6dc60ed86d29d90ef900207d848ad
    Closes-Bug: #1474765