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.
Should use with to make sure the file is also closed if an exception occurs:

```python
with open(full_path, 'r') as fp:
  config.parse(fp)
```