Fix reclaim_instance_interval < 0 never delete instance completely

CONF.reclaim_instance_interval is interval in seconds for reclaiming
deleted instances. It should take effect only when value is greater
than 0. But current when set it with -1 for example, it will pass check
if we should soft delete an instance, but can't pass check whether should
clean soft deleted instance, then never have chance to clean up a soft deleted
instance and release the quota.
This commit sets CONF.reclaim_instance_interval's minimum value to 0.

Closes-Bug: #1518848

Change-Id: Idd8faad1a40f8898a9471504e664c3676b1b0e06

nova/compute/manager.py

@@ -161,8 +161,10 @@ interval_opts = [
                help="Number of seconds between instance network information "
                     "cache updates"),
     cfg.IntOpt('reclaim_instance_interval',
+               min=0,
                default=0,
-               help='Interval in seconds for reclaiming deleted instances'),
+               help='Interval in seconds for reclaiming deleted instances. '
+                    'It takes effect only when value is greater than 0.'),
     cfg.IntOpt('volume_usage_poll_interval',
                default=0,
                help='Interval in seconds for gathering volume usages'),