Make /var/lib/ironic/{,images,master_images} readable by nginx

These directories contain images that are symlinked to /httpboot and have
to be readable by the web server. We haven't had any problems with it only
because the direct deploy does not use /var/lib/ironic with HTTP images.

Change-Id: Idbcc7ed583234ba5d8fd48913146e288e1096bdf

playbooks/roles/bifrost-ironic-install/tasks/bootstrap.yml

@@ -224,7 +224,7 @@
     state: directory
     mode: 0750
     owner: "ironic"
-    group: "ironic"
+    group: "{{ nginx_user }}"
   loop:
     - "/var/lib/ironic"
     - "/var/lib/ironic/master_images"

releasenotes/notes/images-permissions-2042490e3ca13656.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    Makes ``/var/lib/ironic`` and its images subdirectories readable by nginx.
+    This is required for using the images cache.