Update shrink scenario tests

This patch updates the shrink scenario tests to handle with the
"available" or "shrinking_possible_data_loss_error" state received
from the manila share manager after a failed shrink operation when
attempt to shrink a share with a size smaller than thecurrent used
space.

Change-Id: Idf34a149e5a975d5fbced0ec78d102c5d90b87c6
Related-Bug: #1858328
diff --git a/manila_tempest_tests/common/constants.py b/manila_tempest_tests/common/constants.py
index 8791837..69cd1ff 100644
--- a/manila_tempest_tests/common/constants.py
+++ b/manila_tempest_tests/common/constants.py
@@ -20,7 +20,8 @@
 STATUS_CREATING = 'creating'
 STATUS_DELETING = 'deleting'
 STATUS_SHRINKING = 'shrinking'
-
+STATUS_SHRINKING_POSSIBLE_DATA_LOSS_ERROR = (
+    'shrinking_possible_data_loss_error')
 TEMPEST_MANILA_PREFIX = 'tempest-manila'
 
 # Replication