Adds running_deleted_instance_reaper task.

This adds a periodic task to cleanup erroneously running instances. The
impetus of the patch was a XenServer specific issue bug #911366, where deleted
instances would remain running on the host machine.

The patch however is hypervisor agnostic and is generally useful as a
housekeeping task to make sure these 'zombied' instances are detected.

Change-Id: Iddc6a88920a537a3a115f8b9bc0039ec0e24a194

nova/utils.py

@@ -1181,3 +1181,31 @@ def read_cached_file(filename, cache_info):
     cache_info['data'] = data
     cache_info['mtime'] = mtime
     return data
+
+
+@contextlib.contextmanager
+def temporary_mutation(obj, **kwargs):
+    """Temporarily set the attr on a particular object to a given value then
+    revert when finished.
+
+    One use of this is to temporarily set the read_deleted flag on a context
+    object:
+
+        with temporary_mutation(context, read_deleted="yes"):
+            do_something_that_needed_deleted_objects()
+    """
+    NOT_PRESENT = object()
+
+    old_values = {}
+    for attr, new_value in kwargs.items():
+        old_values[attr] = getattr(obj, attr, NOT_PRESENT)
+        setattr(obj, attr, new_value)
+
+    try:
+        yield
+    finally:
+        for attr, old_value in old_values.items():
+            if old_value is NOT_PRESENT:
+                del obj[attr]
+            else:
+                setattr(obj, attr, old_value)