Fixed a volume creation re-schedule error

Don't see any reason to construct a list for request_spec['volume_id'].
And it indeed causes problems when the scheduler receives a reschedule
request with such a request_spec.

Fixes bug #1164857

Change-Id: I811e957d5bf9c814165d55ea0042833ea72c8de5
(cherry picked from commit 46b2a7d)

cinder/volume/manager.py

@@ -368,7 +368,7 @@ def _reschedule(self, context, request_spec, filter_properties,
             LOG.debug(_("No request spec, will not reschedule"))
             return
 
-        request_spec['volume_id'] = [volume_id]
+        request_spec['volume_id'] = volume_id
 
         LOG.debug(_("volume %(volume_id)s: re-scheduling %(method)s "
                     "attempt %(num)d") %