IPA: do not try to add override gid twice
By default user and group overrides use the same attribute name for the
GID and this cause SSSD machinery to add the same value twice which
cause an error in ldb_add() or ldm_modify().
Related to https://fedorahosted.org/sssd/ticket/2514
Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>