BGP: Cleanup interfaces properly on instance delete or exit

BGP: Cleanup interfaces properly on instance delete or exit

Perform interface cleanup as an instance is deleted. This takes care of the
scenario when BGP exits (or is stopped/restarted) too as instances undergo
deletion and the interface cleanup is done as the last step in that.

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>
Fixes: 46abd3e3e6eb1f723ea7905c9919b65f1a76f385
Ticket: CM-9410
Reviewed By: CCR-4143
Testing Done: Reran failed test