Fix variable increment syntax

With bash, the i++ syntax does not quite work the way one might expect.
Use POSIX standard syntax instead, consistent with the rest of the code.

Change-Id: Ic98ee03290dc46f4cfa1284672e9b4f9052739ef

labs/scripts/ubuntu/setup_cinder_volumes.sh

@@ -131,7 +131,7 @@ function check_cinder_services {
             echo "Cinder services seem to be up and running!"
             return 0
         fi
-        ((i++))
+        i=$((i + 1))
         if [[ "$i" -gt "20" ]]; then
             echo "Error, cinder services are not working as expected."
             exit 0
@@ -171,7 +171,7 @@ function wait_for_cinder_volume {
             # The purpose of this method is to resolve cinder-volumes race.
             return 0
         fi
-        ((i++))
+        i=$((i + 1))
         if [[ "$i" -gt "20" ]]; then
             echo "Error creating cinder volume."
             echo "[Warning]: Debug cinder volumes service on the compute node.

labs/scripts/ubuntu/setup_lbaas_controller.sh

@@ -43,7 +43,7 @@ function check_apache_service {
     until service apache2 status | grep 'not running'; do
         sudo service apache2 stop
         sleep 10
-        i ++
+        i=$((i + 1))
         if [ $i -gt 3 ]
         then
             break