Fix IS_ZUUL_FT and make it more succinct

This patch fixes the broken devstack plugin script and slightly changes
the way the var IS_ZUUL_FT is used. When the var is unset, a bash unary
operator expected error should've been emitted because of this part:
${IS_ZUUL_FT+yes}. Unset vs set was a bit confusing so this patch not
only fixes the part but forces the var into always being set and changes
it to "True" vs "False".

Change-Id: I25b418f7776d0d9a00fcf7846a406aba62d40190

devstack/lib/tacker

@@ -258,7 +258,7 @@ function configure_tacker {
     iniset $TACKER_CONF DEFAULT auth_strategy $TACKER_AUTH_STRATEGY
     _tacker_setup_keystone $TACKER_CONF keystone_authtoken
 
-    if [[ "${TACKER_MODE}" == "all" || -v IS_ZUUL_FT ]]; then
+    if [[ "${TACKER_MODE}" == "all" || "${IS_ZUUL_FT}" == "True" ]]; then
         iniset "/$Q_PLUGIN_CONF_FILE" ml2_type_flat flat_networks $PUBLIC_PHYSICAL_NETWORK,$MGMT_PHYS_NET
         iniset "/$Q_PLUGIN_CONF_FILE" ovs bridge_mappings $PUBLIC_PHYSICAL_NETWORK:$PUBLIC_BRIDGE,$MGMT_PHYS_NET:$BR_MGMT
 

devstack/plugin.sh

@@ -41,7 +41,7 @@ if is_service_enabled tacker; then
             tacker_horizon_install
         fi
 
-        if [[ "${TACKER_MODE}" == "all" || -v IS_ZUUL_FT ]]; then
+        if [[ "${TACKER_MODE}" == "all" || "${IS_ZUUL_FT}" == "True" ]]; then
             echo_summary "Setup initial tacker network"
             tacker_create_initial_network
             echo_summary "Check and download images for tacker initial"

devstack/settings

@@ -1,8 +1,9 @@
 TACKER_MODE=${TACKER_MODE:-all}
 USE_BARBICAN=True
 KUBERNETES_VIM=${KUBERNETES_VIM:-False}
+IS_ZUUL_FT=${IS_ZUUL_FT:-False}
 
-if [ "${TACKER_MODE}" == "all" -o ${IS_ZUUL_FT+yes} ]; then
+if [ "${TACKER_MODE}" == "all" -o "${IS_ZUUL_FT}" == "True" ]; then
     MGMT_PHYS_NET=${MGMT_PHYS_NET:-mgmtphysnet0}
     # br-infra is created by devstack multi node job
     # https://opendev.org/zuul/zuul-jobs/src/branch/master/roles/multi-node-bridge