support for testing vrf netns support for L3VPN testing

[pguibert6WIND]

change proposed in https://github.com/pguibert6WIND/topotests/tree/bgp_vrf_to_bgp_vpn-vmerge1-namingvrf does the following:

• at least provide the vrf renaming per machine
• then provide a mechanism to test with vrf with netns.

[rzalamena]

The issue seems to be resolved and I'll close it.

Feel free to re-open it or open a new issue.