cli_driver: Rework driver script to preserve json-embedded whitespace

Previously, the line read from the script was being passed through
python's string.split() and string.join() functions. This was causing
sequences of successive whitespace characters to be collapsed down to
a single space, (in turn causing spurious failures when comparing
against expected output with successive whitespace).

Rework the driver to avoid munging the line past the separation of the
first command word.