Use creates property of exec

... instead of unless + test command, to leverage the built-in feature
and simplify the logic in our own modules.

Change-Id: I0d5e98fcfcada5637dd0c3cc60e503b73241f3fd

manifests/storage/loopback.pp

@@ -62,7 +62,7 @@ define swift::storage::loopback(
   exec { "create_partition-${name}":
     command => "dd if=/dev/zero of=${base_dir}/${name} bs=${byte_size} count=0 seek=${seek}",
     path    => ['/usr/bin/', '/bin'],
-    unless  => "test -f ${base_dir}/${name}",
+    creates => "${base_dir}/${name}",
     before  => Anchor['swift::config::end'],
   }