Use "display_name" in "nova-manage vm list".

Fixes bug 894807.

This patch changes the output of "nova-manage vm list" to use the
display_name for an instance instead of the hostname in the output.
This makes the output consistent with what you get with "nova list".

Change-Id: I3131c5aad8770481bac7731c06110a98b4cc2966

bin/nova-manage

@@ -969,7 +969,7 @@ class VmCommands(object):
         for instance in instances:
             print "%-10s %-15s %-10s %-10s %-26s %-9s %-9s %-9s" \
                   "  %-10s %-10s %-10s %-5d" % (
-                instance['hostname'],
+                instance['display_name'],
                 instance['host'],
                 instance['instance_type'].name,
                 instance['vm_state'],