ci: Add .papr-ex.yaml

ci: Add .papr-ex.yaml

I'd like to start working towards migrating the OSTree CI towards the
new PAPR model running in PACI. In order to take it for a test drive
(pun intended) without yet replacing our current CI, the new PAPR now
knows to look for a `.papr-ex.yaml` file. So as to not conflict with the
current PAPR, no GitHub statuses will be output other than a
`required-ex` label, which plays the same role as `required`. For more
information, see:

https://github.com/projectatomic/papr/commit/85b1d0a91eba6597d01b52b674de38a876f030fe

The `.papr-ex.yaml` version is essentially the same except for:

1. the `FAH27-insttests` suite runs directly in the scheduled container
   (notice the new `kvm: true` key)
2. the `host:` based contexts (rpm-ostree and flatpak) are missing since
   we don't support provisioning a host just yet

Keeping them in sync could get annoying, though building confidence in
the new approach will allow us to migrate faster.

Closes: #1587
Approved by: cgwalters