Always check for expanded path of secret file

Previously if you were using relative path for secret file, then following could happen:

```ruby
# .... --secret-file "~/.ssh/secret_file" \
File.exist?("~/.ssh.secret") #=> false
```

Also check this issue for knife-digital_ocean: https://github.com/rmoriz/knife-digital_ocean/issues/42

With this PR expanded path is always used in order to check if file exists.

you're doing some voodoo with nil.to_s which is both hidden and, i think, incorrect since File.expand_path(nil.to_s) returns the pwd.  would prefer something like this:

```ruby
            secret_file_path = knife_config[:secret_file] || @chef_config[:encrypted_data_bag_secret]
            if secret_file_path
             expanded_secret_file_path = File.expand_path(secret_file_path)
             IO.read(expanded_secret_file_path) if Fille.exist?(expanded_secret_file_path)
           end
```