fix unit tests + add graph method ...

* fix unit tests on roles (adding mandatory description)
* adding a method to get roles graph in json

simplify roles configuration (backend groups)

change backend key name in attributes + methods ...

* adding get_backends
* adding get_selfattributes (attributes users can change themselves)
* adding unit tests
* rename backend-attributes -> bakends (for homogeneity with role.yml)

implementing nesting roles

fixing typos in role files

adding test cases and reworking test env