Switch devstack / tempest testing to fedora-27
Switch devstack / tempest testing to fedora-27

We have fedora-27 nodes online, and want to remove fedora-26. Convert
this jobs to fedora-27.

Change-Id: Iee7e0cae4345a8c49bd0134a3d716610a2d6babc
Depends-On: https://review.openstack.org/547107
Signed-off-by: Paul Belanger <e7139c8e6c4ea75cc1c287ce081b2dc0f3ab39ec@redhat.com>

openstack-infra/project-config,openstack-infra/project-config