backup v2_key before creating a new one

Xav and the community seem very concerned about loosing the encryption key (`v2_key`). And if you don't know what you are doing, it is pretty easy to blow it away.

The goal here is to backup the previous encryption key so mistakes are not so costly.

IT also makes the tools more consistent. `auth_tool` blows up if a key exists while `appliance_console_cli` happily overwrites it. The `appliance_console` nicely prompts you with a "are you sure".

Since nothing is lost, adding a force option or "are you sure" is not necessary. Did decide to leave it in the `appliance_console`. That tool is expected to have more hand holding.

https://bugzilla.redhat.com/show_bug.cgi?id=1148606