Port ansible doc for plugins to use DOCUMENTATION variables

* Using docstrings conflicts with the standard use of docstrings
* PYTHON_OPTIMIZE=2 will omit docstrings.  Using docstrings makes future
  changes to the plugin and module code subject to the requirement that we
  ensure it won't be run with optimization.

lib/ansible/plugins/connection/lxd.py

@@ -2,8 +2,10 @@
 # (c) 2017 Ansible Project
 # GNU General Public License v3.0+ (see COPYING or https://www.gnu.org/licenses/gpl-3.0.txt)
 
-"""
-DOCUMENTATION:
+from __future__ import (absolute_import, division, print_function)
+__metaclass__ = type
+
+DOCUMENTATION = """
     author: Matt Clay <matt@mystile.com>
     connection: lxd
     short_description: Run tasks in lxc containers via lxc CLI
@@ -27,9 +29,6 @@ DOCUMENTATION:
             - name: ansible_lxd_executable
 """
 
-from __future__ import (absolute_import, division, print_function)
-__metaclass__ = type
-
 import os
 from distutils.spawn import find_executable
 from subprocess import call, Popen, PIPE