gen-hydra-key should not overwrite key if file already exists

[v0d1ch]

Why

I was bitten by this recently - when generating hydra keys hydra-node should not overwrite existing keys.

What

Detect if the file path we are providing is already present/existing and want the user about this.

How

Alter hydra-node executable gen-hydra-key to look at the provided file path and warn if a file already exists.