Always check for expanded path of secret file

Previously if you were using relative path for secret file, then following could happen:

```
# .... --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.