implementing weight and normalizing each kind of attributes

implement search

adding method and unit tests to get all the attributes of one backend

adding get_attributes

better unit tests

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)

better checks for attributes

adding checks for mandatory elements of attributes.yml

begin attributes handling manipulation