topotests: add vpnv4 ecmp test

topotests: add vpnv4 ecmp test

This test ensures that BGP VRF instance is able to import ECMP
paths, and is able to install 2 labelled routes accordingly.

The test also ensures that the imported 172.31.0.10/32 prefix
is selected and that the reason why the 172.31.0.10/32 prefix is
selected is not 'Locally configured route'. Actually, imported
routes do not figure out correctly the peer, and the reason is
falsely mentioned as local.

This test also uses IP ranges used for documentation and for
testing.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>