bgpd/ospfd: make bgp and ospf json response a bit more consistent

bgpd/ospfd: make bgp and ospf json response a bit more consistent

Problem reported that some bgp and ospf json commands did not return
any json output at all if the bgp/ospf instance did not exist.
Additionally, some bgp and ospf json commands did not return any json
output if the instance existed but no neighbors were defined.  This
fix makes these commands more consistent in returning empty braces for
json output and issue a message if not using json output.  Additionally,
made the flag "use_json" a bool to make it consistent since previously,
it had been defined as an int, char, u_char, and bool at various places.

Ticket: CM-21040
Signed-off-by: Don Slice <dslice@cumulusnetworks.com>