Find unused TCP port for client proxy

We currently use hardcoded port number and this causes a problem when we
run multiple FAFT instance on the same host. Let's change this to find
an unused port whenever we want to start a client proxy.

BUG=chrome-os-partner:19097
TEST=Run FAFT and see port number changes everytime.

Change-Id: I37e410a386d37dbd2800bcf15568a1a6dd21e087
Signed-off-by: Vic Yang <victoryang@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/49840
Reviewed-by: Tom Wai-Hong Tam <waihong@chromium.org>
1 file changed