'tahoe create-introducer' should create introducer.furl immediately

[davidsarah]

A minor usability issue when setting up a grid is that the introducer needs to be started once before it creates the introducer.furl to copy to the config of other nodes. This is documented, but it requires users to pay close attention to the order of operations in the docs. introducer.furl could just as well be created by tahoe create-introducer, which would relax this order. I ran into this earlier today, and know of several other users running into it.