Use total reserved quota value instead of partial

While reserving snapshot we reserve gigabytes too. In case of
group snapshot we reserve the total gigabytes but in case of an
exception we print just the last volume gb info which needs to be
changed.

Change-Id: Ibc8b1c5a967d05b3e52f992a1e76e74c9120e3f1
Closes-Bug: 1728457

cinder/volume/api.py

@@ -979,9 +979,11 @@ class API(base.Base):
                             total_reserve_opts[key] + value
             reservations = QUOTAS.reserve(context, **total_reserve_opts)
         except exception.OverQuota as e:
-            quota_utils.process_reserve_over_quota(context, e,
-                                                   resource='snapshots',
-                                                   size=volume.size)
+            quota_utils.process_reserve_over_quota(
+                context,
+                e,
+                resource='snapshots',
+                size=total_reserve_opts.get('gigabytes', volume.size))
 
         return reservations