Do not remove member attributes while updating others

Because of a missing check member attributes (for use with action: member)
are cleared when a non-member attribute is changed. The fix simply adds a
check for None (parameter not set) to gen_add_del_lists in
ansible_freeipa_module to make sure that the parameter is only changed if
it should be changed.

All places where the add and removal lists have been generated manually
have been changed to also use gen_add_del_lists.

Resolves: #252 (The "Manager" attribute is removed when updating any user
                attribute)