Added Get User List methods to win_client and linux_client

* Added get_user_list method to windows_client which uses the winrm
  remote client to run a powershell command to get user objects,
  parse them into dictionaries and make a list of the usernames
  on the target instance
* Added get_user_list method to linux_client which reads the
  /etc/passwd file on the target instance and returns a list of
  the usernames.

Change-Id: Ifeb408929d598fdff82a7f4f8c0f0912ff8fd1f4
Merge "Added Get User List methods to win_client and linux_client"