Fix incorrect Glance image metadata description

For each image, when user clicks on Edit Image --> Update Metadata, the list
of various available metadata categories and properties is displayed in a
pop-up window. For each category and property, a user-friendly description is
displayed at the bottom of the pop-up.

The description for "Common Operating System Properties" category says - "When
adding an image to Glance, you may specify some common image properties that
may prove useful to consumers of your image." This is too generic and seems
like a copy-paste of the description from glance-common-image-props.json that
the author later failed to update.

Fixing this to show a more meaningful message.

Change-Id: I5477b1702932e07ab1fd499204d3ff1348abba3b
Closes-Bug: #1483063

etc/metadefs/operating-system.json

@@ -1,10 +1,10 @@
 {
     "display_name": "Common Operating System Properties",
     "namespace": "OS::OperatingSystem",
-    "description": "When adding an image to Glance, you may specify some common image properties that may prove useful to consumers of your image.",
+    "description": "Details of the operating system contained within this image as well as common operating system properties that can be set on a VM instance created from this image.",
     "protected": true,
     "resource_type_associations" : [
-	    {
+        {
             "name": "OS::Glance::Image"
         },
         {