warn if v2_key exists with meaningful error

fix_auth (the most common place to generate a key) tries to provide
a user context that they should not just remove the file.

The cli is more powerful, so a --force option is provided

- cli only replaces v2_key if --force
- cli does replace v2_key if fetching from remote
- fix_auth displays warning. User must backup key first

/cc @Fryguy @jrafanie 
Good news. The cli is run by automation / power users while the fix_auth is a general tool. So don't want to go to deep on this discussion.

I want to deprecate fix_auth after we can move it into the appliance_console (and cli). When we reorganize it, I would like to introduce sub commands.

Do we want to spend a lot of time with `--force`?
I'd prefer just removing the option all together if this takes too much time.