Fix user module under python3 (#4560)

Using something like:

    - name: Create ssh keys
      user:
        name: root
        generate_ssh_key: yes
      register: key

result into this traceback on F24

    Traceback (most recent call last):
      File \"/tmp/ansible_jm5d4vlh/ansible_module_user.py\", line 2170, in <module>
        main()
      File \"/tmp/ansible_jm5d4vlh/ansible_module_user.py\", line 2108, in main
        (rc, out, err) = user.modify_user()
      File \"/tmp/ansible_jm5d4vlh/ansible_module_user.py\", line 660, in modify_user
        return self.modify_user_usermod()
      File \"/tmp/ansible_jm5d4vlh/ansible_module_user.py\", line 417, in modify_user_usermod
        has_append = self._check_usermod_append()
      File \"/tmp/ansible_jm5d4vlh/ansible_module_user.py\", line 405, in _check_usermod_append
        lines = helpout.split('\\n')
    TypeError: a bytes-like object is required, not 'str'

lib/ansible/modules/system/user.py

@@ -220,6 +220,7 @@ import grp
 import platform
 import socket
 import time
+from ansible.module_utils._text import to_native
 
 try:
     import spwd
@@ -401,7 +402,7 @@ class User(object):
         helpout = data1 + data2
 
         # check if --append exists
-        lines = helpout.split('\n')
+        lines = to_native(helpout).split('\n')
         for line in lines:
             if line.strip().startswith('-a, --append'):
                 return True