Change extra_size from hardcoded 1 to CONF.volume.volume_size_extend

test_volume_from_snapshot has extra_size hardcoded to 1 and it expects
the returned size increased by 1GB. This does not work with PowerFlex
cinder driver because PowerFlex/VxFlex OS requires volumes to be sized
in multiples of 8GB. Even if 1GB is given, 8GB will be actually used
when powerflex_round_volume_capacity = True.

Closes-Bug: #1949927
Change-Id: I04c3d6145fbc201355d38a7a058a37791227f733
diff --git a/tempest/api/volume/test_volumes_snapshots.py b/tempest/api/volume/test_volumes_snapshots.py
index fd2e7c4..a58da7e 100644
--- a/tempest/api/volume/test_volumes_snapshots.py
+++ b/tempest/api/volume/test_volumes_snapshots.py
@@ -167,7 +167,8 @@
     @decorators.idempotent_id('677863d1-3142-456d-b6ac-9924f667a7f4')
     def test_volume_from_snapshot(self):
         """Test creating volume from snapshot with extending size"""
-        self._create_volume_from_snapshot(extra_size=1)
+        self._create_volume_from_snapshot(
+            extra_size=CONF.volume.volume_size_extend)
 
     @decorators.idempotent_id('053d8870-8282-4fff-9dbb-99cb58bb5e0a')
     def test_volume_from_snapshot_no_size(self):