Remove the redundant check for 'os-migrate_volume_completion'

[zhangyanzi]

Remove the redundant KeyError check for 'os-migrate_volume_completion'
The fact that this extension is registered as WSGI action implies that the body must contain a key with the same name as action.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/58434

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/58434
Committed: https://git.openstack.org/cgit/openstack/cinder/commit/?id=a0693a91f55b2b08a46f1396dd1a1102783df9c6
Submitter: Jenkins
Branch: master

commit a0693a91f55b2b08a46f1396dd1a1102783df9c6
Author: zhangyanzi <email address hidden>
Date: Tue Nov 26 12:19:48 2013 +0800

    Redundant check in os-migrate_volume_completion

    Remove the redundant check for 'migrate_volume_completion', and add the UT
    for it.

    Change-Id: I6b643dbf9ae1302cafe6748a361123d7d76c437f
    Closes-Bug: #1254963