servod: system config map & input_type

This CL reshuffles the logic inside resolve_val in system_config to
allow for mapped values to also leverage input_type.
This means that if for
dut-control random_ctrl:val
|val| maps to "1.2" then the float 1.2 will be passed to the set method.

It also starts the process of adding unittests to system_config.py

BUG=chromium:806148, chromium:812152
TEST=manual testing

Change-Id: I1a4590b22b8d7003454d51b040cfa4ba5105c7c7
Reviewed-on: https://chromium-review.googlesource.com/1168270
Commit-Ready: Ruben Rodriguez Buchillon <coconutruben@chromium.org>
Tested-by: Ruben Rodriguez Buchillon <coconutruben@chromium.org>
Reviewed-by: Todd Broch <tbroch@chromium.org>
2 files changed