Ensure reactor listens for signals on the correct object path

[Barry Warsaw]

While analyzing LP: #1413265 I discovered a spot where the object_path wasn't being passed into the signal reactor, so in the UDM case, two reactors could be active at the same time. In that case, a 'finished' signal could be incorrectly caught by both reactors, causing one to exit early and then not finding its expected destination files.

This bug tracks the fix in si so that the object_path is always passed into the signal reactor, ensuring that the 'finished' signal coming from UDM gets passed to the correct reactor.