Chutney's single-onion-ipv6-md test fails on master

When we merged legacy/trac#27215 (moved), it changed the default HiddenServiceVersion from 2 to 3.

Chutney expects the default to be v2, so single-onion-ipv6-md fails with an error, because IPv6-only v3 single onion services are not yet supported.

I have a patch to fix this issue. I'll attach it to this ticket.