Fix getting metadata attr error in snapshot-list command

Because of 'start_version' parameter in decorator here [0]
we won't have metadata attr in args using api versions from 3.0 to 3.21.

This patch changes getting metadata from args logic for compatibility with
the api versions 3.0 - 3.21.

[0] - b73b393240/cinderclient/v3/shell.py (L1237)

Change-Id: I4aa099556c57c49e9ad74fe80e5591d738cf3aa0
Closes-Bug: #1659561

cinderclient/v3/shell.py

@@ -1245,14 +1245,20 @@ def do_snapshot_list(cs, args):
     if args.display_name is not None:
         args.name = args.display_name
 
+    metadata = None
+    try:
+        if args.metadata:
+            metadata = shell_utils.extract_metadata(args)
+    except AttributeError:
+        pass
+
     search_opts = {
         'all_tenants': all_tenants,
         'name': args.name,
         'status': args.status,
         'volume_id': args.volume_id,
         'project_id': args.tenant,
-        'metadata': shell_utils.extract_metadata(args)
-        if args.metadata else None,
+        'metadata': metadata
     }
 
     snapshots = cs.volume_snapshots.list(search_opts=search_opts,
@@ -1263,4 +1269,4 @@ def do_snapshot_list(cs, args):
     sortby_index = None if args.sort else 0
     utils.print_list(snapshots,
                      ['ID', 'Volume ID', 'Status', 'Name', 'Size'],
-                     sortby_index=sortby_index)
+                     sortby_index=sortby_index)