exabgp: use `next-hop self` feature just added today

It is now possible to use `next-hop self` from a process.
exabgp: MED is like a metric, a lower MED has a higher priority

Therefore, change `--decrease` to `--increase` to invert default MED
values for up and down.
exabgp: add a blackhole for ExaBGP controlled range

This way, when an IP address is unavailable, we don't loop it
everywhere. Also add a blackhole for the internal network for the same
purpose.

This is not a real change since our edge router does not have a
default route.
exabgp: update lab schema with more indications