PROXY: Share common code of save_{group,user}()
These two functions (save_user() and save_group()) share, between
themselves, the code preparing the attributes that are going to be
stored in the sysdb.
This patch basically splits this code out of those functions and
introduces the new prepare_attrs_for_saving_ops().
Related:
https://fedorahosted.org/sssd/ticket/3134
Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>