Rewrite link parsing for finding v2 schemas

What we called 'links' are no longer returned in a container of
objects, they are top-level entity attribtues. This fixes the
parsing of the entities to look in the correct place when trying
to locate a specific schema.

Add a helper for printing to stderr and exiting with a non-zero
exit code.

Map 'name' to 'Attribute' when explaining a schema.

Related to bp glance-client-v2

Change-Id: Ib98e912a7af0bb570b4fd738733edd9b837d1a05

glanceclient/v2/shell.py

@@ -14,6 +14,7 @@
 #    under the License.
 
 from glanceclient.common import utils
+from glanceclient import exc
 
 
 def do_image_list(gc, args):
@@ -23,9 +24,14 @@ def do_image_list(gc, args):
     utils.print_list(images, columns)
 
 
-@utils.arg('name', metavar='<NAME>', help='Name of model to describe.')
+@utils.arg('model', metavar='<MODEL>', help='Name of model to describe.')
 def do_explain(gc, args):
     """Describe a specific model."""
-    schema = gc.schemas.get(args.name)
-    columns = ['Name', 'Description']
-    utils.print_list(schema.properties, columns)
+    try:
+        schema = gc.schemas.get(args.model)
+    except exc.SchemaNotFound:
+        utils.exit('Unable to find requested model \'%s\'' % args.model)
+    else:
+        formatters = {'Attribute': lambda m: m.name}
+        columns = ['Attribute', 'Description']
+        utils.print_list(schema.properties, columns, formatters)