Unity: Use thin clone when cloning volume (#9)

Unity storage now supports cloning volume by taking its snapshot,
it's more efficient than previous implementation.

However, there is limitation on the count of thin clones inside one LUN
family sharing the same base LUN. Every time the limitation reaches, a
new volume will be cloned using `dd` copy. Then thin clone will be taken
from the new dd-copied volume.

DocImpact
Implements: blueprint unity-fast-clone

Change-Id: I5e4264ce9917e831e3efe27dc332641c9f94c07f
(cherry picked from commit a6c2223)

Author: peter-wangxu

cinder/tests/unit/volume/drivers/dell_emc/unity/fake_exception.py

@@ -46,6 +46,10 @@ class UnityNothingToModifyError(StoropsException):
     pass
 
 
+class UnityThinCloneLimitExceededError(StoropsException):
+    pass
+
+
 class ExtendLunError(Exception):
     pass