Improve user and group management on OpenWrt

These improvements are the results of extensive local testing with rex-build. Generally the aim was to enhance user and group management support on OpenWrt while trying to avoid increasing code complexity by using as most standard/common solutions as possible. Mainly:
* use passwd instead of chpasswd for setting user passwords
* use groupdel and userdel installed from repo and through Rex::User::Linux instead of trying to replicate their functionality
* add Rex::User::OpenWrt::user_list
* various minor fixes

Please review the proposed changes and let me know if something needs to be modified.

ps.: I also prepared a changeset for rex-build to enable tests for OpenWrt related to user/group management