Add logging for external commands

Specifically, this ends up logging the GPU installer command, the uname
command that gets the arch, the installation path remount, and the
nvidia-persistenced commands.

BUG=None
TEST=compile cos-extensions and install gpu drivers

Change-Id: I7fe05461d40ce9538a876f79b671ce5a885175e7
Reviewed-on: https://cos-review.googlesource.com/c/cos/cos-extensions/+/141685
Reviewed-by: Angel Adetula <angeladetula@google.com>
Tested-by: Kevin Berry <kpberry@google.com>