Module usergroup

new system module called usergroup
This module manages the user supplementary groups using the usermod commandline utility. The code is based on user moddule.
Right now there is only Generic platform support tested on the
- linux (debian, ubuntu)
- solaris (openindiana)

The usage of the plugin
- add user to the supplementary groups /also supported by user module/
- remove user from the supplementary groups /not supported by user module/
- set exactly user supplemenary groups /also supported by user module/
- remove user from all supplementary groups /not sure if user module support this/