remote_build: Simpler interface

As per the discussions in https://github.com/autotest/virt-test/pull/1594 and https://github.com/autotest/tp-qemu/pull/73 , I'm adding this as a pull request to simplify commenting before going home for the weekend.
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Update the remote_build interface to make it figure out the default
values of lots of input parameters without being explicitly told. This
change would mean that this:

    username = params.get("username", "")
    password = params.get("password", "")
    address = vm.get_address(0)
    source_dir = data_dir.get_deps_dir("clock_getres")
    build_dir = params.get("build_dir", "/tmp")
    shell_client = params.get("shell_client", "ssh")
    file_transfer_client = params.get("file_transfer_client", "scp")
    shell_prompt = params.get("shell_prompt")
    shell_linesep = params.get("shell_linesep")
    builder = remote_build.Builder(address, source_dir, client=shell_client,
                                   file_transfer_client=file_transfer_client,
                                   username=username,
                                   password=password,
                                   build_dir=build_dir,
                                   prompt=shell_prompt,
                                   linesep=shell_linesep)

..can now be done like this, given that params is set:

    address = vm.get_address(0)
    source_dir = data_dir.get_deps_dir("clock_getres")
    builder = remote_build.Builder(params, address, source_dir)

The idea is to reduce the boiler plate needed by test case designer to
fetch all the parameters from the cartesian configuration. It also fixes
the lack of a separate file_transfer_port, which is needed by the rss
file transfer client.

Signed-off-by: Jonas Eriksson <jonas.eriksson@enea.com>