Create structure of flows' packages

Create structure of flows' packages

1. Create_volume flows were moved to separate files from __init__.py.
   In future each flow should be implemented in a separate file.

2. Add flows.common package for tasks that could be used by different flows.

3. Proposed file structure for flows is more clean and helps to find and reuse
common code.

4. _exception_to_unicode function and tests were removed because never used.

Partially implements: blueprint create-volume-flow

Change-Id: I63473f549f0c501fe0f373830bc1080239d01892