fix PeerGroup AFI handling
fix PeerGroup AFI handling

This fixes a bug that the duplicated AFI is configured with the peer
group. OverwriteNeighborConfigWithPeerGroup() appends the peer group's
AFI configuration to the existing one instead overwriting.

Signed-off-by: FUJITA Tomonori <93dac1fe9c4b2a3957982200319981492ad4976e@gmail.com>

fujita/gobgp,a16/gobgp,a16/gobgp,j-kato/gobgp,j-kato/gobgp,a16/gobgp,osrg/gobgp,osrg/gobgp,a16/gobgp,j-kato/gobgp,a16/gobgp,osrg/gobgp,fujita/gobgp,fujita/gobgp,fujita/gobgp,a16/gobgp,a16/gobgp,osrg/gobgp