Don't expect replies from mute cells

Broadcast messages were failing if a cell stopped responding. We now
only send broadcast messages that need replies to cells that are not
mute.

Closes-bug: #1312468
Change-Id: If242e44535d455894b23af37d6c4aa3d09bb5395

Conflicts:
	nova/cells/utils.py
Catch mute cells straight away when attempting to send a message

Change-Id: I151d97dec578631a69e42bc5fbe298fa1e3a4836
Revert initial last_seen to min, also add argument to match
parent for _send_json_responses

Change-Id: Ifaf9e81d1a33386b67575624358514efbd9b75e3