CI system changes

["Colin Walters" <walters verbum org>]

As part of the recent organizational changes in how Red Hat is using libostree:
https://mail.gnome.org/archives/ostree-list/2019-August/msg00003.html

We are winding down our previous CI system:
https://github.com/projectatomic/papr/issues/105

There's no one replacement for some of the "test contexts" (e.g. we're not dropping Travis), but we are using 
OpenShift Prow to both execute some tests and most importantly as a "merge bot".

If you have commit access to libostree, this means you should take a quick look at some of the Prow docs:
https://prow.k8s.io/command-help
A lot more at https://github.com/kubernetes/test-infra/tree/master/prow

In general, you basically need to know

/approve - This PR looks like a good idea, is sane
/lgtm - This code is ready to go

A main benefit of the "merge bot" logic is that (just like our previous use of Homu 
https://github.com/servo/homu) you can drop /lgtm and the PR will merge when (and only if) tests are green.

This is only part of a larger effort around CI we're doing in the CoreOS group - right now I'm working on a 
CI setup that uses nested virt in GCP:
https://github.com/coreos/coreos-assembler/blob/b35c722a3c106ccf7c1eb2788089e9d9ec2197a9/doc/openshift-gcp-nested-virt.md
And once we have that we can fully decomission the previous PAPR instances, and it should be *way* easier to 
write useful and interesting tests for libostree.