Make sure target exists before running it

By generating the targets list once prior to running the targets, there
is the possibility that a script doesn't exist anymore or doesn't have
the right permissions by the time it is called.

See for instance [1] that deletes files from the target_dir.

This commit ensures the target exists and is executable before trying to
run it.

[1] 41ee9895d5/elements/hiera/os-refresh-config/configure.d/10-hiera-disable
Change-Id: I746b5c89082fedc01b32ad00e573a25f8c4201c0

bin/dib-run-parts

@@ -116,7 +116,9 @@ PROFILE_DIR=$(mktemp -d --tmpdir profiledir.XXXXXX)
         output "Running $target_dir/$target"
         target_tag=${target//\//_}
         date +%s.%N > $PROFILE_DIR/start_$target_tag
-        $target_dir/$target
+        if [ -x $target_dir/$target ]; then
+            $target_dir/$target
+        fi
         target_tag=${target//\//_}
         date +%s.%N > $PROFILE_DIR/stop_$target_tag
         output "$target completed"