Enable image rotation for Magnum

In case, when there's more then 1 image with name `fedora-coreos-latest`
Magnum fails to resolve it's uuid and fails to create a template from such
image.

In order to prevent this happening, we do enable image rotation in CI
to fix upgrade jobs.

Changing visability to `community` does hide the image from non-owner
unless explicitly asked for listing community images.

Change-Id: I1d5f02824e0c0fed820ee0808127efccad1017cf

tests/roles/bootstrap-host/templates/user_variables_magnum.yml.j2

@@ -22,6 +22,8 @@ magnum_glance_images:
     properties:
       os_distro: "fedora-coreos"
     checksum: "da359b10f9aa165c4f81e6cd9ca5f81b"
+    hide_method: community
+    keep_copies: 1
 
 magnum_cluster_templates:
   - name: k8s