When internal operations are performed (e.g., best-path selection, next-hop

When internal operations are performed (e.g., best-path selection, next-hop
change processing etc.) that refer to the BGP instance, the correct BGP
instance must be referenced and not the default BGP instance. The default
BGP instance is the first instance on the instance list. In a scenario
where one BGP instance is deleted (through operator action such as a
"no router bgp" command) and another instance exists or is created, there
may still be events in-flight that need to be processed against the
deleted instance. Trying to process these against the default instance
is erroneous. The calls to bgp_get_default() must be limited to the user
interface (vtysh) context.

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>