Tempest: add a Cinder extend_with_snapshot feature flag

A new tempest test is being added in https://review.openstack.org/#/c/200108/
but it doesn't run by default because the test fails on Juno. So a
feature flag in Tempest is added. This patch turns on this feature
flag at Devstack's side.

Change-Id: If1cf90dac3edc81a483fc51da74495042c96d543
diff --git a/lib/tempest b/lib/tempest
index fb4b0d3..85e8d5f 100644
--- a/lib/tempest
+++ b/lib/tempest
@@ -477,6 +477,8 @@
     # Volume
     # TODO(dkranz): Remove the bootable flag when Juno is end of life.
     iniset $TEMPEST_CONFIG volume-feature-enabled bootable True
+    # TODO(jordanP): Remove the extend_with_snapshot flag when Juno is end of life.
+    iniset $TEMPEST_CONFIG volume-feature-enabled extend_with_snapshot True
 
     local volume_api_extensions=${VOLUME_API_EXTENSIONS:-"all"}
     if [[ ! -z "$DISABLE_VOLUME_API_EXTENSIONS" ]]; then