Expose toml contents and location via http

[stuarteberg]

Currently, if I'm curious about how a given DVID server is configured, I have to ssh into the machine where it's running, figure out where the toml file is located (not always easy), and inspect it.

It would be nice if DVID exposed an endpoint such as .../api/server/config or .../api/server/toml which returns the complete toml file contents (including comments, etc.), and also showed the filepath to the toml file (say, as a comment inserted at the top of the file).