update test_volume_boot_pattern to assign one value

In commit 6809bab06e5b29efe02c11b9f4e53707c26eb1f9, the
create_snapshot function was changed to return one value.
The test_volume_boot_pattern is currently being skipped because of
a bug, so this was noticed during local testing with the test
enabled.

Related to blueprint clients-return-one-value

Change-Id: I18a68a0a4d25c8f965664d04d83dbb0842fb7037
diff --git a/tempest/scenario/test_volume_boot_pattern.py b/tempest/scenario/test_volume_boot_pattern.py
index c584a6e..910fd79 100644
--- a/tempest/scenario/test_volume_boot_pattern.py
+++ b/tempest/scenario/test_volume_boot_pattern.py
@@ -66,7 +66,7 @@
 
     def _create_snapshot_from_volume(self, vol_id):
         snap_name = data_utils.rand_name('snapshot')
-        _, snap = self.snapshots_client.create_snapshot(
+        snap = self.snapshots_client.create_snapshot(
             volume_id=vol_id,
             force=True,
             display_name=snap_name)