horizon: set FORCE_GENERATE to no if not set

Signed-off-by: Christian Berendt <[email protected]>
Change-Id: I303ee5367aba0ae114e52b7b559faf06133f2304
(cherry picked from commit b2d8883)

Author: Christian Berendt

docker/horizon/extend_start.sh

@@ -2,7 +2,7 @@
 
 set -o errexit
 
-FORCE_GENERATE="${FORCE_GENERATE}"
+FORCE_GENERATE="${FORCE_GENERATE:-no}"
 HASH_PATH=/var/lib/kolla/.settings.md5sum.txt
 
 if [[ ${KOLLA_INSTALL_TYPE} == "binary" ]]; then