ansible_freeipa_module.py: New gen add and intersection list functions

Two new functions have been added for member management in plugins:

gen_add_list(user_list, res_list)
    Generate the add list for addition of new members.

gen_intersection_list(user_list, res_list)
    Generate the intersection list for removal of existing members.

gen_add_list should be used to add new members with action: members and
state: present. It is returning the difference of the user and res list
if the user list is not None.

gen_intersection_list should be used to remove existing members with
action: members and state: absent. It is returning the intersection of
the user and res list if the user list is not None.