cmd/gomote: consistently support a direct-to-buildlet mode, bypassing coordinator

For testing new builder images, this adds support to gomote to connect
to a buildlet directly, without going via the coordinator. If the
builder name contains a '@' character, the name is expected to be of
the form <build-config-name>@ip[:port].

Also update the gomote run -path docs while I'm here, since I always
forget the magic expansions (which at least are documentd in the
buildlet godoc). Copy them to the flags doc.

Change-Id: I50227855897106e1cc60c1881700866d5fddf2a5
Reviewed-on: https://go-review.googlesource.com/38775
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
9 files changed