Restore execution of plugin tests
Restore execution of plugin tests

When commit 52634f55fdab470492ed68095e8f465e173a909c changed the
directory structure, it also deleted blazar/tests/plugins/__init__.py.
As a result, all tests under blazar/tests/plugins/ where skipped. This
patch restores this file to resume executing plugin tests.

Change-Id: I47eec42fd1d51d21c6abc53100b8785c766bf863

ChameleonCloud/blazar,openstack/blazar,openstack/blazar,ChameleonCloud/blazar,stackforge/blazar,stackforge/blazar