Add details for backup fail reason

[huangtianhua]

1. create a stack has a cinder volume resource
2. stack in CREATE_COMPLETE
3. snapshot the stack
4. the stack snapshot failed, want to get the reason by run the command 'heat stack-show':

    stack_status_reason | Resource SNAPSHOT failed: Error: error

5. The reason of no worth, and there is details in cinder if backup failed, so we should get the reason and set it to stack_status_reason.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/177633

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/177633
Committed: https://git.openstack.org/cgit/openstack/heat/commit/?id=3cff77bef330b0f323045920d2124ccb5bd45b8e
Submitter: Jenkins
Branch: master

commit 3cff77bef330b0f323045920d2124ccb5bd45b8e
Author: huangtianhua <email address hidden>
Date: Mon Apr 27 10:08:43 2015 +0800

    Add details of volume backup failed reason

    Set the failed reason details when snapshot failed for
    cinder volume, then user can get the worth information to
    know why the snapshot failed.

    Change-Id: I1001c41550e5147b96df0846180b587b3c3849cd
    Closes-Bug: #1448820