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.

src/allmydata/client.py

@@ -19,7 +19,6 @@ from allmydata.introducer import IntroducerClient
 
 class Client(node.Node, Referenceable):
     implements(RIClient)
-    CERTFILE = "client.pem"
     PORTNUMFILE = "client.port"
     STOREDIR = 'storage'
     NODETYPE = "client"