Add f20 experimental job to tempest

test_stamp_pattern.py is skipped since long,
but I was not able to reproduce the issue locally
on f20. Would be nice to see does the issue happens
on the gate vms with f20.

[1] https://review.openstack.org/#/c/52740/

Change-Id: I1ea3dfc7dcd1c0ec207b13a988852c6d5795037f
Merge "Add f20 experimental job to tempest"