Added DEFAULT_EXECUTABLE as a constant

Use DEFAULT_EXECUTABLE when no executable is passed to
_low_level_command_exec

Works as a standard constant - can be overridden in all the normal ways
and defaults to /bin/sh

Motiviation is for a user that only has /bin/bash in /etc/sudoers - my real-life test case fails when _low_level_command_exec defaults to /bin/sh and succeeds when I set executable=/bin/bash in my ansible config file