disable v2 by default

[xiang90]

etcd operator does not support backup v2 data.

we do not have a good story on v2 yet. probably we should disable v2 by default. and warn users v2 backup is not supported when they try to enable it.

[raoofm]

@xiang90 if etcd-io/etcd#6925 is implemented then yes we dont need v2, else it is very imp to support both v2 and v3.

[hongchaodeng]

We can use this flag:
https://github.com/coreos/etcd/blob/master/Documentation/op-guide/configuration.md#--enable-v2