buildlet: add config file to alter username
buildlet: add config file to alter username

My gomote username is josharian.
My whoami is josh.
As a result, I need to run 'gomote -user=josharian'
every time, which is an annoyance.

Add a simple config file to allow a user to change their user name.
On startup, gomote checks whether user-$(whoami).user exists.
If so, it reads it to get the real username.
So I have a config file ~/.config/gomote/user-josh.user
with the contents 'josharian'.

It's a bit of an ugly hack, but it works and spares me some pain.

Change-Id: I372d5a786b99c9e3c6a57f25b0a38b9146f23598
Reviewed-on: https://go-review.googlesource.com/42230
Run-TryBot: Josh Bleecher Snyder <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/build,golang/build,golang/build,golang/build