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

lib/ansible/runner/__init__.py

@@ -527,7 +527,7 @@ class Runner(object):
         ''' execute a command string over SSH, return the output '''
 
         if executable is None:
-            executable = '/bin/sh'
+            executable = C.DEFAULT_EXECUTABLE
 
         sudo_user = self.sudo_user
         rc, stdin, stdout, stderr = conn.exec_command(cmd, tmp, sudo_user, sudoable=sudoable, executable=executable)