Add missing test case for unshelving a volume backed instance

[Pranali Deore]

Unshelving a volume backed instance was not working before merging the patch [1].

In case of volume backed instance, snapshot is not taken when an instance is shelved,
so shelve_image_id key is not set to the instance system metadata.
If shelve_image_id is None, UnshelveException was raised which was incorrect.

In current tempest suit there is no such testcase for unshleving a volume backed
instance.

[1] https://review.openstack.org/#/c/144582/

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/215480

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/215480
Committed: https://git.openstack.org/cgit/openstack/tempest/commit/?id=9fe5eb311bebfbf11ae9158800e297d00dcb436d
Submitter: Jenkins
Branch: master

commit 9fe5eb311bebfbf11ae9158800e297d00dcb436d
Author: PranaliDeore <email address hidden>
Date: Wed Jul 22 05:28:00 2015 -0700

    Add shelve/unshelve volume backed instance test

    Added missing testcase for unshelving a volume backed instance.

    Closes-Bug: 1476622
    Depends-On: Ic023853dbd787f0f29877d6135adb7299a87abdb
    Change-Id: I46422a0829323340bc95213ba90b22163b9520f0