Update executor nodes to F27

[jlebon]

We were previously stuck on F25 because of issues with docker cp: https://bugzilla.redhat.com/show_bug.cgi?id=1489505.

But now, this is actively preventing newer features to be used. E.g. for ostreedev/ostree#1462, I couldn't get oci-kvm-hook to work on F25, but it did work on F27. Checking the reproducer in rhbz#1489505, it looks like the docker cp issue isn't there anymore, so we're no longer blocked.