Bug-4827: BGP Add-Path OpenConfig Support

Bug-4827: BGP Add-Path OpenConfig Support

Added mapping from BGP RIB and Peer Module
to OpenConfig API to support BGP Add-Path configuration.
In "Neighbor" configuration per AFI/SAFI Add-Path
capability is exposed (enabled).
In "Global" configuration per AFI/SAFI path selection
strategy is exposed (enabled+maximum paths).

For the sake of gathering info about path selection modes,
expose "N" value in AddPathBestNPathSelection.

Change-Id: Icedecb0e9d18e6fd3b29bfd28d49172209bfbe37
Signed-off-by: Milos Fabian <milfabia@cisco.com>