Low: daemons: Use the peer value pased to attrd_peer_message.

The various attrd IPC functions that tools like attrd_updater use call
create_attrd_op to construct the XML IPC request. That function may
cause an attribute like src="attrd_updater" to be added to the request.
This is fine for incoming IPC requests.

However, pacemaker-attrd in turn takes those request messages and
rebroadcasts them across the cluster as peer messages. In that case,
the src="attrd_updater" attribute causes messages to be logged
indicating that there's no attrd_updater cluster node.

We're already passing the peer to attrd_peer_message, so just use that
instead of looking at the request XML.