Change mode of DATA_DIR to 0755 as well

It looks libvirt/qemu requires mode 0755 to functioning properly,
but DATA_DIR won't be set to 0755 if it is different from default.

Change-Id: I37ca0b02b6a75b3756860b547e84c37ccfc99d13
Closes-Bug: #1677421
diff --git a/stack.sh b/stack.sh
index 20cdc1d..310676a 100755
--- a/stack.sh
+++ b/stack.sh
@@ -328,6 +328,7 @@
 DATA_DIR=${DATA_DIR:-${DEST}/data}
 sudo mkdir -p $DATA_DIR
 safe_chown -R $STACK_USER $DATA_DIR
+safe_chmod 0755 $DATA_DIR
 
 # Configure proper hostname
 # Certain services such as rabbitmq require that the local hostname resolves