Make dpm command output more test friendly

Added 'Success' at the beginning of the command output every time
it didn't fail, so that we can easily check the output in tests.
The command is brand new, only used by us on CTS tests, so this
shouldn't break anybody.

Bug: 17312478
Change-Id: Ie67bf5b2f44e658e7009e5bd87f8cb81be515584