[tests] murano coverage job can lead to false positives because it checkouts stale code

[Kirill Zaitsev]

See this commit as an example. https://review.openstack.org/#/c/352088/2 http://logs.openstack.org/88/352088/2/check/murano-coverage-db-ubuntu-xenial/bf7db82/

Since the job does checkout — it can checkout stale code (the one that differs from up-to-date master). In the current master we could have increased the coverage, and the job would compare the result with stale coverage results. Thus the job will give false-negatives on legitimate commits.

We could force the job to cherry-pick the commit in question on top of master (or merge, or rebase) in order to get correct results