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
Documented executable setting for ansible.cfg
Merge pull request #2565 from willthames/default_exec

Added DEFAULT_EXECUTABLE as a constant