Seed an empty /nsm/kernelrepo so the manager repo is always valid

so-repo-sync only populates /nsm/kernelrepo after the highstate, so on a
manager the file:///nsm/kernelrepo repo could be assigned before any
repodata exists, failing every dnf op. Run createrepo on the dir when
repodata/repomd.xml is missing, leaving a synced repo untouched.

salt/manager/init.sls

@@ -96,6 +96,18 @@ kernelrepo_dir:
       - group
     - show_changes: False
 
+# Ensure /nsm/kernelrepo is always a valid (if empty) repo before it is ever assigned to
+# a client. Without repodata/repomd.xml an enabled file:///nsm/kernelrepo repo makes every
+# dnf operation fail; so-repo-sync only populates it after the highstate, so seed an empty
+# repo here. Only runs when repodata is missing, so it won't clobber a synced repo.
+kernelrepo_init_empty:
+  cmd.run:
+    - name: createrepo /nsm/kernelrepo
+    - unless: 'test -e /nsm/kernelrepo/repodata/repomd.xml'
+    - require:
+      - file: kernelrepo_dir
+      - pkg: install_createrepo
+
 manager_sbin:
   file.recurse:
     - name: /usr/sbin