Force glance recheck for kernel/ramdisk on rebuild

When doing a rebuild, clear the instance_info's kernel
and ramdisk values, to force the deploy driver to check
glance. This is necessary as those values may have changed
without us knowing.

closes-bug: #1401501

Change-Id: I8d7831789d21d3a2dcafd614989cf1e53dbedd70

ironic/conductor/manager.py

@@ -642,6 +642,12 @@ class ConductorManager(periodic_task.PeriodicTasks):
 
             if rebuild:
                 event = 'rebuild'
+
+                # Note(gilliard) Clear these to force the driver to
+                # check whether they have been changed in glance
+                node.instance_info.pop('kernel', None)
+                node.instance_info.pop('ramdisk', None)
+                node.save()
             else:
                 event = 'deploy'