Fix image_id KeyError in test_create_backup

image_id is added to the result of create_backup, but it's not
added in the header, it's added in body instead, so we can't
get image_id from resp.response, otherwise it will raise
"KeyError: 'image_id'". This is to fix it.

Ref: https://developer.openstack.org/api-ref/compute/#create-server-back-up-createbackup-action

Implements blueprint: clear-deprecated-api

Change-Id: If924bc417c24e5bcc18577d20023f277653d7dd3
1 file changed