allow ansible_ssh_private_key_file to be templated (thanks to benno)

It can be very useful to select the SSH private key file based on inventory information (for example with Amazon EC2 where each host can have a different private key but it's always found in the inventory as ec2_key_name if using the EC2 dynamic inventory script). 

This allows one to do things like this e.g. in group variables:
```
ansible_ssh_private_key_file: "/home/ansible/.ssh/{{ ec2_key_name }}.pem"
```

This pull request implements that (thanks to benno).