Make matrix rooms private

[andrevmatos]

Problem Definition

Right now, matrix p2p communication rooms are public for debugging purposes. long-standing in-code TODO:

raiden/raiden/network/transport/matrix.py

Lines 673 to 677 in b2e2d35

	room = self._client.create_room(
	room_name,
	invitees=[user.user_id for user in invitees],
	is_public=True, # FIXME: debug only
	)

This issue tracks making it private, to avoid possible attackers eavesdropping p2p communication and e.g. getting the exchanged Secret beforehand.

Out of #2302