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/constants.py

@@ -95,6 +95,7 @@ DEFAULT_SUDO_EXE          = get_config(p, DEFAULTS, 'sudo_exe', 'ANSIBLE_SUDO_EX
 DEFAULT_SUDO_FLAGS        = get_config(p, DEFAULTS, 'sudo_flags', 'ANSIBLE_SUDO_FLAGS', '-H')
 DEFAULT_HASH_BEHAVIOUR    = get_config(p, DEFAULTS, 'hash_behaviour', 'ANSIBLE_HASH_BEHAVIOUR', 'replace')
 DEFAULT_JINJA2_EXTENSIONS = get_config(p, DEFAULTS, 'jinja2_extensions', 'ANSIBLE_JINJA2_EXTENSIONS', None)
+DEFAULT_EXECUTABLE        = get_config(p, DEFAULTS, 'executable', 'ANSIBLE_EXECUTABLE', '/bin/sh')
 
 DEFAULT_ACTION_PLUGIN_PATH     = shell_expand_path(get_config(p, DEFAULTS, 'action_plugins',     'ANSIBLE_ACTION_PLUGINS', '/usr/share/ansible_plugins/action_plugins'))
 DEFAULT_CALLBACK_PLUGIN_PATH   = shell_expand_path(get_config(p, DEFAULTS, 'callback_plugins',   'ANSIBLE_CALLBACK_PLUGINS', '/usr/share/ansible_plugins/callback_plugins'))

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)