Fix handling of parameters that are not set by any flavour for eval_run_d.

flavours/devstack/block-device-size.d/10-size

@@ -2,7 +2,6 @@
 # Add the stack user we recommend folk use.
 
 set -e
-set -x
 
 if (( '16' '>' $IMAGE_SIZE )); then
    echo IMAGE_SIZE=16

lib/common-functions

@@ -40,6 +40,8 @@ function check_flavour () {
 function eval_run_d () {
   local TEMP=`run_d $1`
   echo "$TEMP"
+  if [ `echo "$TEMP" | grep -s "$2"` ]; then
       TEMP=`echo "$TEMP" | grep "$2"`
       eval "$TEMP"
+  fi
 }