cmd/buildlet, cmd/coordinate: add buildlet --reverse-type flag, document better

And change the protocol between the buildlet and coordinator to let
buildlets explicitly declare their host type (the new way) rather than
figuring it out implicitly from their specified builder names, which
are otherwise ignored.

Change-Id: I533cc4517996c264a88af34c65f8b00db3e39975
Reviewed-on: https://go-review.googlesource.com/39205
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
3 files changed