Support a "no_server" flag in test config.

* Will use a running instance of rippled (possibly in a debugger).
* Modify all tests to respect the server_default value.
* Fail test if new account already exists and has a balance.
* README.md with instructions for advanced test debugging, particularly using no_server.

Tentative reviewers @sublimator, @vinniefalco.
So I read through the code a couple of times but I'm still not quite sure what "m" is supposed to be (I see you're copying from above).  Any ideas?  Can we give it a better name or put a comment in?