Migrate ejabberd uploads to mod_s3_upload and Garage

In addition to installing and configuring the new module, this also
enables public access to the S3 API via `bucket-name.s3.kosmos.org` as
well as Web access on `bucket-name.web.s3.kosmos.org` (when enabled).

Also includes some drive-by improvements to Chef attribute naming and
usage.

Co-authored-by: Greg Karékinian <greg@karekinian.com>

site-cookbooks/kosmos-ejabberd/templates/vhost.yml.erb

@@ -21,5 +21,5 @@ host_config:
 
 append_host_config:
   "<%= @host[:name] %>":
-    <%= @host[:append_host_config].chomp %>
+<%= @host[:append_host_config].chomp %>