It happens that we have some leftovers instances running in an "error"
state (the error comes from the OpenStack scheduled deletion). This
leads to instance creation error during the test because quota limits
are hit.
Let's clean-up everything before running the new tests.
This won't impact tests from other channels as OpenStack is limited to
one CI job at a time.
Signed-off-by: Mathieu Tortuyaux <mtortuyaux@microsoft.com>