etcd: enable v2 support for various tests

[tormath1]

In this PR, we prepare the landing of flatcar-archive/coreos-overlay#1179 which brings etcd/v3 by default.

• flannel does not support yet etcd/v3 so we enable v2 support
• locksmith is WIP to support etcd/v3
• cl.etcd-member.v2-backup-restore test the actual behavior of v2 backup so it requires v2 to be enabled

v2 support is done through this option: https://etcd.io/docs/v3.5/op-guide/configuration/#--enable-v2

Testing done

Tested against the current alpha and the image provided in flatcar-archive/coreos-overlay#1179 (arm64 in progress...)

sudo ./bin/kola run ... cl.etcd-member.v2-backup-restore cl.locksmith.cluster coreos.locksmith.tls cl.flannel.udp cl.flannel.vxlan