RDNBRD: Change default distance of imported table routes to 15

RDNBRD: Change default distance of imported table routes to 15

Signed-off-by: Daniel Walton <dwalton@cumulusnetworks.com>
Reviewed-by:   Donald Sharp <sharpd@cumulusnetworks.com>
Ticket: CM-9898

The 'redistribute neighbor' feature is the primary use case for
importing table routes.  We need the redistribute neighbor routes to
have a lower admin distance than eBGP so that the local table routes are
preferred (if the host is dual homed we could also learn about it via eBGP).