Split workstation-base from "atomic"

This is obviously confusing but right now "ostree workstation" is really
"Atomic". I'm not changing the refs or the names of the manifest JSON files yet,
but the high level idea here is we have:

 - workstatoin-base (should be like comps...that's another issue)
 - atomic (inherits desktop-base, adds rpm-ostree + container tooling)
   Also I added a compat symlink so pungi doesn't need an immediate change.

fedora-atomic-workstation.json

@@ -0,0 +1,15 @@
+{
+    "ref": "fedora/rawhide/x86_64/workstation",
+
+    "include": "fedora-workstation-base.json",
+
+    "automatic_version_prefix": "28",
+    "mutate-os-release": "28",
+
+    "packages": [
+        "atomic",
+        "rpm-ostree",
+        "skopeo",
+        "docker"
+    ]
+}