Fix grenade runs which don't use tempest

A bug was reported for a grenade job that doesn't use tempest. Fix is to
check for existence of tempest.conf file before copying it.

Change-Id: I3a79347c8d5b84ea756e7571d0216384fbf2a05e
Closes-Bug: #1651767

upgrade-tempest

@@ -78,9 +78,15 @@ if $(source $BASE_DEVSTACK_DIR/stackrc; is_service_enabled neutron); then
     Q_L3_ENABLED="True"
 fi
 
+if [[ -f $BASE_RELEASE_DIR/tempest/etc/tempest.conf ]]; then
+    # Use the same config to target smoke tests.
+    cp $BASE_RELEASE_DIR/tempest/etc/tempest.conf $TARGET_RELEASE_DIR/tempest/etc/tempest.conf
+else
+    configure_tempest
+fi
+
 install_tempest_plugins
-# Use the same config to target smoke tests.
-cp $BASE_RELEASE_DIR/tempest/etc/tempest.conf $TARGET_RELEASE_DIR/tempest/etc/tempest.conf
+
 DEST=$DEST_BACKUP
 
 set +o xtrace