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 46b2a7dd3224c4ed00457520b95569442386d273)
Merge "Fixed a volume creation re-schedule error" into stable/grizzly