Fix up variable substitution

This seems to work undefined varibles and null strings

Author: jovial

bin/rally-verify-wrapper.sh

@@ -13,12 +13,12 @@ artifacts_dir=/home/rally/artifacts
 tempest_source_default="--source /opt/tempest"
 
 tempest_source="$tempest_source_default"
-if [ ! -z ${TEMPEST_SOURCE-x} ]; then
+if [ ! -z ${TEMPEST_SOURCE:+x} ]; then
     tempest_source="--source $TEMPEST_SOURCE"
 fi
 
 tempest_version=""
-if [ ! -z ${TEMPEST_VERSION-x} ]; then
+if [ ! -z ${TEMPEST_VERSION:+x} ]; then
     tempest_version="--version $TEMPEST_VERSION"
 fi
 
@@ -34,12 +34,12 @@ if [ -f ~/tempest-skip-list ]; then
 fi
 
 pattern=""
-if [ ! -z ${TEMPEST_PATTERN-x} ]; then
+if [ ! -z ${TEMPEST_PATTERN:+x} ]; then
    pattern="--pattern $TEMPEST_PATTERN"
 fi
 
 concurrency=""
-if [ ! -z ${$TEMPEST_CONCURRENCY-x} ]; then
+if [ ! -z ${$TEMPEST_CONCURRENCY:+x} ]; then
    concurrency="--concurrency $TEMPEST_CONCURRENCY"
 fi