Fix unit tests to detect import failures

This commit adds onto the existing unit test which simply runs
'testr list-tests' to run discovery on tempest. This is done to ensure
that from a bare tempest repo with the requirements all present we can
actually import and list all the tests. However, there wasn't a check
of the return code. Historically this has been an issue with discovery
failing so adding a check to enforce it moving forward should prevent
it from slipping back in. Part of this is to also fix the test so that
it runs discovery on the correct test suite.

In order to make the unit test pass a config dependency in the
scenario test test_basic_server_ops during the testscenarios call in
load_tests had to be removed. Instead if there is a configuration
error raised while running load tests on this module instead of
failing it will just return the base test class. This is fine
because the test itself will fail on the same configuration
dependencies that load_tests is, however it won't happen silently
during discovery anymore.

Change-Id: Ie74c020de7c9c27adc4ff68dddbae5d4481d1224
Merge "Fix unit tests to detect import failures"