create-node: avoid introducer.furl=None

Previously, "tahoe create-node" without an --introducer= argument would
result in the literal string "None" being written into tahoe.cfg:

 [client]
 introducer.furl = None

We were using config.get("introducer",""), but that didn't suffice because
the key was actually present: it just had a value of None, which then got
stringified into "None" when writing out tahoe.cfg.

This briefly caused test/cli/test_create to fail, as the startup code tried
to parse "None" as a FURL. This only happened against a development version
of Foolscap which accidentally became sensitive to unparseable FURLs in
started Reconnectors. I fixed that in the final foolscap-0.12.5 release, so
we shouldn't hit this bug, but I wanted to fix it properly in the tahoe-side
source.

src/allmydata/client.py

@@ -256,6 +256,10 @@ class Client(node.Node, pollmixin.PollMixin):
 
         # read furl from tahoe.cfg
         tahoe_cfg_introducer_furl = self.get_config("client", "introducer.furl", None)
+        if tahoe_cfg_introducer_furl == "None":
+            raise ValueError("tahoe.cfg has invalid 'introducer.furl = None':"
+                             " to disable it, use 'introducer.furl ='"
+                             " or omit the key entirely")
         if tahoe_cfg_introducer_furl:
             introducers[u'default'] = {'furl':tahoe_cfg_introducer_furl}
 

src/allmydata/scripts/create_node.py

@@ -279,7 +279,8 @@ def write_node_config(c, config):
 def write_client_config(c, config):
     c.write("[client]\n")
     c.write("# Which services should this client connect to?\n")
-    c.write("introducer.furl = %s\n" % config.get("introducer", ""))
+    introducer = config.get("introducer", None) or ""
+    c.write("introducer.furl = %s\n" % introducer)
     c.write("helper.furl =\n")
     c.write("#stats_gatherer.furl =\n")
     c.write("\n")