Rework BGP configuration

Rework BGP configuration

This patch reworks the structure of BGP configuration so that it follows
the logical organization of the RIB.

The core change is the creation of BGPPeer as a full-blown configuration
item. This encapsulates the previous proposal configuration and has the
inverse relationship with the RIB: the RIB is its dependency and it
pulls things like the local AS and BGP identifier from it.

Futher improvement is the introduction of bgp-table-type, which allows
explicit listing of tables which should be advertized in the BGP Open
message as well as specification of which tables should be tracked in
the local RIB.

Change-Id: I905759a3d9e780394579238d48ff0aee009a5544
Signed-off-by: Robert Varga <rovarga@cisco.com>
Signed-off-by: Maros Marsalek <mmarsale@cisco.com>