bgp: add Flat method on AddrPrefixInterface
bgp: add Flat method on AddrPrefixInterface

This method returns a map[string]string which can be used to create
structured outputs, specialized by implemented types.

fujita/gobgp,Dipsingh/gobgp,ramrunner/gobgp,osrg/gobgp,ishidawataru/gobgp,tamihiro/gobgp,a16/gobgp,osrg/gobgp,osrg/gobgp,fujita/gobgp,a16/gobgp,ramrunner/gobgp,ishidawataru/gobgp,j-kato/gobgp,a16/gobgp,a16/gobgp,a16/gobgp,osrg/gobgp,a16/gobgp,tamihiro/gobgp,a16/gobgp,tamihiro/gobgp,j-kato/gobgp,ramrunner/gobgp,fujita/gobgp,fujita/gobgp,Dipsingh/gobgp,j-kato/gobgp,ishidawataru/gobgp,Dipsingh/gobgp,tamihiro/gobgp