Make it easier to find a network generated during 'make test-network-all'
When we run a tor 'make test-network-all' we create several chutney networks in quick succession. If one of them fails, it's very hard to locate its logs.
So instead of moving the 'nodes' directory to a timestamped backup path when the next network is created, let's create the nodes directory at a timestamped path, and then link the latest timestamped directory to nodes.
Then we can log the unchanging path, so users can find the logs later.