bgpd: Strip Route Targets during VRF-to-VRF route leak

bgpd: Strip Route Targets during VRF-to-VRF route leak

During VRF-to-VRF route leaking, strip any extraneous route targets. This
ensures that source-VRF-specific route targets or route targets that are
internally assigned for the VRF-to-VRF route leaking don't get attached
to the route in the target VRF.

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>
Reviewed-by:   Donald Sharp <sharpd@cumulusnetworks.com>
Reviewed-by:   Don Slice <dslice@cumulusnetworks.com>