node.py: use 'node.pem' for all nodes

node.py: use 'node.pem' for all nodes

Rather than use separate client.pem and introducer.pem files, use 'node.pem'
for all nodes regardless of what type it is. This is slightly cleaner, but
introduces a compatibility. Users who upgrade to this change should do
'mv client.pem node.pem' to avoid generating a new certificate and thus
changing their TubID.