openshift/os: simplify image definitions

First, stop using the `fcos-buildroot` image. It's no longer defined in
this repo and we don't strictly need it.

Instead, use cosa as the buildroot. This then allows us to get rid of
the redundant `base_images` definition, and use the `root` image as our
`from`.

Use `FROM overridden` in the Dockerfile literal to make it clear that
it's always going to be overridden anyway.

Author: jlebon

ci-operator/config/openshift/os/openshift-os-master.yaml

@@ -1,17 +1,11 @@
-base_images:
-  coreos_coreos-assembler_latest:
-    name: coreos-assembler
-    namespace: coreos
-    tag: latest
 build_root:
   image_stream_tag:
-    name: fcos-buildroot
+    name: coreos-assembler
     namespace: coreos
-    tag: testing-devel
+    tag: latest
 images:
 - dockerfile_literal: |
-    # COSA + source code
-    FROM registry.ci.openshift.org/coreos/coreos-assembler:latest
+    FROM overridden
     WORKDIR /src
     # Tell the build scripts that we have setup a directory for unprivileged build
     ENV COSA_DIR=/tmp/cosa
@@ -29,10 +23,7 @@ images:
     # Go back to unprivileged user for COSA
     USER builder
     WORKDIR /tmp/cosa
-  inputs:
-    coreos_coreos-assembler_latest:
-      as:
-      - registry.ci.openshift.org/coreos/coreos-assembler:latest
+  from: root
   to: build-image
 promotion:
   to: