fix(tests): report curl output if it wasn't "Powered by Deis"

If the smoke test fails, it now reports what curl does see to the
test runner for help in debugging failures.

The smoke test now registers with the router rather than directly
to the controller at port 8000, and the .HostName config field was
more appropriately named to .Domain.

Fixes #1417.

TESTING: `export DEIS_EXAMPLE_APP=helloworld` (or another that doesn't end with "Powered by Deis") and run `make test-smoke`.
Yes, it's not in there by default on any of the build nodes so we intend to `ssh-add` it each time. (We also have to ensure `ssh-agent` is running, because it isn't by default when we ssh in, at least on the Mac node.)

So I think the logic here was intended to be "create the key if needed, but either way make sure it's been ssh-add'ed."