Don't run the periodic task if ticks_between_runs is below zero

if the user sets say image_cache_manager_interval=-1, then
don't add the task to the list of periodic tasks

remove extra braces that i had added

DocImpact

Fixes LP #1084232

Change-Id: Ieecd67ddbc70b815a88f40e72ca2899787d75988

nova/manager.py

@@ -125,8 +125,9 @@ def __init__(cls, names, bases, dict_):
             if getattr(value, '_periodic_task', False):
                 task = value
                 name = task.__name__
-                cls._periodic_tasks.append((name, task))
-                cls._ticks_to_skip[name] = task._ticks_between_runs
+                if task._ticks_between_runs >= 0:
+                    cls._periodic_tasks.append((name, task))
+                    cls._ticks_to_skip[name] = task._ticks_between_runs
 
 
 class Manager(base.Base):