Make ssh lookup a tad more robust

If hostname is missing, as it typically is when your ~/.ssh/config
contains a wildcard entry, _sshconf_lkup() would bail out with an
exception. We now handle the lack of hostname in a more robust fashion,
just like is already done for port.

Problem looks like this;
```
kll@lingloi320 ~/kod/py-junos-eznc $ cat ~/.ssh/config
Host *
 ServerAliveInterval 120
kll@lingloi320 ~/kod/ROUMIGA $ python roumiga.py 
Password: 
Traceback (most recent call last):
  File "roumiga.py", line 17, in <module>
    dev = Device(host='my_host_or_ipaddr', user='jeremy', password='jeremy123' )
  File "../py-junos-eznc/lib/jnpr/junos/device.py", line 209, in __init__
    self._sshconf_lkup()
  File "../py-junos-eznc/lib/jnpr/junos/device.py", line 182, in _sshconf_lkup
    self._hostname = found['hostname']
KeyError: 'hostname'
```

Now fixed! =)