travis: add a test for a yum-based pipeline

Travis uses Ubuntu, which does not ship dnf, so introduce a yum
stage that allows us to test actual generation of trees on Travis.

We use this to generate a tree containing the tools necessary to
create abritrary Fedora-based build images in the future. We base
this on Fedora 27, as that is the last version that is installable
using yum rather than dnf.

In the future, once we support pipelines with nested build-images,
rather than just using the host OS as the build image, this will
allow us to bootstrap arbitrary pipelines on Travis.

Signed-off-by: Tom Gundersen <teg@jklm.no>

samples/build-from-yum.json

@@ -0,0 +1,22 @@
+{
+  "name": "build-from-yum",
+  "stages": [
+    {
+      "name": "org.osbuild.yum",
+      "options": {
+        "releasever": "27",
+        "repos": {
+          "fedora": {
+            "name": "Fedora",
+            "baseurl": "https://archives.fedoraproject.org/pub/archive/fedora/linux/releases/$releasever/Everything/$basearch/os/",
+            "gpgkey": "file:///etc/pki/rpm-gpg/RPM-GPG-KEY-fedora-$releasever-$basearch"
+          }
+        },
+        "packages": [
+          "dnf",
+          "systemd"
+        ]
+      }
+    }
+  ]
+}