Make exclude task vars comply to property format

The should look like this: SNF_IMAGE_PROPERTY_<TASK_NAME>_TASK

snf-image-helper/common.sh

@@ -164,7 +164,7 @@ check_if_excluded() {
 
     test "$PROGNAME" = "snf-image-helper" && return 0
 
-    eval local do_exclude=\$SNF_IMAGE_EXCLUDE_${PROGNAME:2}_TASK
+    eval local do_exclude=\$SNF_IMAGE_PROPERTY_EXCLUDE_${PROGNAME:2}_TASK
     if [ -n "$do_exclude" ]; then
         warn "Task $PROGNAME was excluded and will not run."
         exit 0

snf-image-helper/snf-image-helper.in

@@ -88,7 +88,7 @@ fi
 # in case it is left mounted...
 trap '{ umount "$target"; }' ERR
 
-if [ -z "$SNF_IMAGE_EXCLUDE_ALL_TASKS" ]; then
+if [ -z "$SNF_IMAGE_PROPERTY_EXCLUDE_ALL_TASKS" ]; then
     # Redirect standard error to standard output,
     # prepend a timestamp before each line of output.
     echo "Execute all snf-image tasks...."