Add mirror keyring dep

Change-Id: Ib5a12cf4e4b3f6fe55bf0bb889201b186536e45e
diff --git a/maas/map.jinja b/maas/map.jinja
index bcf8a48..6a0a201 100644
--- a/maas/map.jinja
+++ b/maas/map.jinja
@@ -29,6 +29,7 @@
 Debian:
   pkgs:
   - simplestreams
+  - ubuntu-cloudimage-keyring
 {%- endload %}
 
 {%- set mirror = salt['grains.filter_by'](mirror_defaults, merge=salt['pillar.get']('maas:mirror', {})) %}