BGP: VRF registration and cleanup

BGP: VRF registration and cleanup

Various changes and fixes related to VRF registration, deletion,
BGP exit etc.

- Define instance type
- Ensure proper handling upon instance create, delete and
  VRF add/delete from zebra
- Cleanup upon bgp_exit()
- Ensure messages are not sent to zebra for unknown VRFs

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>
Reviewed-by:   Donald Sharp <sharpd@cumulusnetworks.com>
Ticket: CM-9128, CM-7203
Reviewed By: CCR-4098
Testing Done: Manual