FRR: Publishing a new service causes the bgp session to flake

[fedepaol]

Because of how the current frr template looks like, when we move from 0 services to 1 or more services (or viceversa), the neighbour is added only to the ipfamily section related to the service. When we have no advertisements for a neighbor, it's added to both (see

metallb/internal/bgp/frr/config.go

Line 114 in 8377501

{{- if eq (len .Advertisements) 0}}

)

This causes the session to be recreated.