bgpd: Register for label only the default instance

bgpd: Register for label only the default instance

Ensure that the registration for the "in" label for a unicast prefix
is done only in the default instance. The zebra label manager as well
as other code in BGP only has support for assigning labels in the
default instance.

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>
Reviewed-by:   Daniel Walton <dwalton@cumulusnetworks.com>
Reviewed-by:   Donald Sharp <sharpd@cumulusnetworks.com>
Ticket: CM-17110
Reviewed By: CCR-6588
Testing Done: Manual tests, mpls