[qa] Fix sync_blocks timeout argument

Motivation for this change is mainly to make sync_blocks behavior easier to
understand. Behavior is unchanged in the normal case when there are only 2
nodes in the rpc_connections set. When there are more than 2 nodes, the
previous "timeout -= wait" statement wouldn't take into account time spent
waiting for all nodes and as a result could lead to blocking for longer than
the requested timeout.