Fix map usage on py3 in v2 test_volumes_snapshots_list

This commit fixes the usage around map in
test_snapshot_list_param_marker(). In py2 map() returns a list, but in
py3 map() returns an map iterator. To work around this difference simply
use a list comprehension instead.

Change-Id: I0219ee7ac35d7389263af67eb0dde2b0f96ea22f
diff --git a/tempest/api/volume/v2/test_volumes_snapshots_list.py b/tempest/api/volume/v2/test_volumes_snapshots_list.py
index 8f68322..f389b59 100644
--- a/tempest/api/volume/v2/test_volumes_snapshots_list.py
+++ b/tempest/api/volume/v2/test_volumes_snapshots_list.py
@@ -13,8 +13,6 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
-import operator
-
 from tempest.api.volume import base
 from tempest import config
 from tempest import test
@@ -89,7 +87,7 @@
         # The list of snapshots should end before the provided marker
         params = {'marker': self.snapshot_id_list[1]}
         snap_list = self.snapshots_client.list_snapshots(**params)['snapshots']
-        fetched_list_id = map(operator.itemgetter('id'), snap_list)
+        fetched_list_id = [snap['id'] for snap in snap_list]
         # Verify the list of snapshots ends before the provided
         # marker(second snapshot), therefore only the first snapshot
         # should displayed.