Refactor cinder/tests/test_volume.py

Refactor test_volume.py to remove code duplication.
- remove iterant definition of the 'expected' dict from
'test_create_delete_volume' and 'test_create_delete_snapshot'
method;
- move 'test_copy_volume_to_image_status_available',
'test_copy_volume_to_image_status_use' and
'test_copy_volume_to_image_exception' methods to new class
 CopyVolumeToImageTestCase;
- move 'test_volume_get_active_by_window' and
'test_snapshot_get_active_by_window' to new class
GetActiveByWindowTestCase;
- create base class BaseVolumeTestCase for TestCase classes.
bp cinder-tests-improvement

Change-Id: I5d3644375d321e613418ea8e544557d33401ff77