Description of problem:
Cinder retype command fails if volume doesn't have a source type.
Version-Release number of selected component (if applicable):
We discovered it in OSP 5.0 but it seems that this issue can
happen even in the master branch.
100% on OSP 5.0
Steps to Reproduce:
1. Make sure that cinder.conf doesn't specify a default volume type
2. Create volume without specifying volume type
3. Retype this volume to the new type
Volume should get retyped
1. Configuring a default volume type in cinder.conf resolves that issue.
2. Use cinder migrate instead of retype if all you want is to move volume between different backends.
The patch is merged for Mitaka upstream.