Issue LIBCLOUD-499: Add support for ssh_config and proxycommand

since 1.10 paramiko supports ssh_config parsing and proxycommand.

Proxycommand is essential if you have a ssh gateway between you and your computes. Some big infrastructures does.
Just to be on the safe side, we should probably do:

```python
if not os.path.exists(full_path) or os.path.isdir(full_path)
...
```