This decorator can be used to temporarily mark some tests as unstable.
This may help sometimes to debug such test which is failing often
but not always in the gate because it will still be run but will
not cause all job failure when it fails.
This may be also used to easily track in logstash how often
such test is failing by looking for describption of unstability
reason set in decorator.
Change-Id: I79ce70f479506ec2b3216747d533ff2e450685aa
Related-Bug: #1813198