Bug 6788: peer singleton service closed just after initialization

Bug 6788: peer singleton service closed just after initialization

In case of HA failover scenario, configured peer is not getting initiated on
new singleton group master node, as the neighbor is removed from configuration when
default example-rib-impl is initiated (OpenConfig BGP configuration written to config DS).
To avoid overriding neighbors configuration after failover, minimize scope of rib-impl
representation from "protocol" to "global" node.

Change-Id: If4df2a070e0cbceae1c42cbe0d0c5a2258388092
Signed-off-by: Milos Fabian <milfabia@cisco.com>