pimd: make the json output a bit more machine-friendly

Mixing well-known and variable property names makes the output difficult
to parse. so wrapped variable-keyed dicts with well-known property
names (such as "oil") in the following outputs -
"show ip mroute json"
"show ip msdp mesh-group json"

Signed-off-by: Anuradha Karuppiah <anuradhak@cumulusnetworks.com>
Reviewed-by: Donald Sharp <sharpd@cumulusnetworks.com>
Reviewed-by: Daniel Walton <dwalton@cumulusnetworks.com>