enabled sftp private key support (instead of password)

this is the code to make #9182 work
The whole logic here is just unnecessarily complex. It should be a single block handling the private key.
```
if (!empty($params['usesshkey']))
{
    // Get the key file name from the config into a variable
    // Check the existance of the key file, etc etc.
    // Create the RSA object
    // Load the key file into RSA object
}
```