Fix the logic of if used with is_service_enabled

The value to be evaluated is the returned value, not the output of the
command.

Change-Id: I22d7c967e911bcfee6b1910f666dbbc647c00085
diff --git a/lib/tempest b/lib/tempest
index b491bf8..048f346 100644
--- a/lib/tempest
+++ b/lib/tempest
@@ -317,7 +317,7 @@
     # set the equiv validation option here as well to ensure they are
     # in sync. They shouldn't be separate options.
     iniset $TEMPEST_CONFIG validation connect_method $ssh_connect_method
-    if [[ ! $(is_service_enabled n-cell) && ! $(is_service_enabled neutron) ]]; then
+    if ! is_service_enabled n-cell && ! is_service_enabled neutron; then
         iniset $TEMPEST_CONFIG compute fixed_network_name $PRIVATE_NETWORK_NAME
     fi