And add a test that indirectly verifies that the BuildConfig and
HostConfig are JSON serializable. They weren't due to an exported func.

But that exported func shouldn't be exported, so unexport it and move
more policy into dashboard/builders.go. (There's been a number of
recent cleanup CLs to move all policy into dashboard/builders.go
instead of sprinkled all over the coordinator)

A future CL will use this JSON in gomote create.

Updates golang/go#30929

Change-Id: I726eaf6a4f3eeaab27d31e2642cb7642111ccd67
Reviewed-on: https://go-review.googlesource.com/c/build/+/168341
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

While debugging golang/go#25386 I found that the firewall defaulting
to on made debugging modules hard. So make gomote default to
no outbound firewall and make it opt-in for people who
want to reproduce the builder more.

Also in this CL: start to use server's builder config, not local state
(follow up to CL 169678 which was incomplete). It's still incomplete
in this CL, but there's now a panic with more useful information to
users telling them to update their binary.

Updates golang/go#30929

Change-Id: I17bded71919af1e7a9181866a1349eb72da40051
Reviewed-on: https://go-review.googlesource.com/c/build/+/170397
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>