test_cases.md

Reduce the test cases from 15 to 9, bearing in mind that:
* Disable weave/coreos gate unless its deployment fixed
* If debian/centos7 fails with net plugin X, ubuntu-xenial/rhel-7 will
  likely fail as well
* Canal also covers the flannel plugin deployment, but keep at least one
  of the flannel plugin deployment, unless it's superseded and removed.
* Keep at least one of each OS/plugin family to be tested in the separate
  nodes layout
* Keep at least one of each OS family to be tested against each of the
  plugin types in default nodes layout
* Rebalance GCE regions for instances, replace asia to eu/us as they
  are the longest running jobs.

Signed-off-by: Bogdan Dobrelya <bdobrelia@mirantis.com>