assh cleanups / default user handling

```
$ AWS_DEFAULT_USER=ubuntu assh datanode
Please select which node you wish to use:

0) metrics_facing-stg-v2-datanode-11 (i-30XXXXX, m1.large, 2014-02-12T20:46:29.000Z)
1) metrics_facing-stg-v2-datanode-12 (i-91XXXXX, m1.large, 2014-02-13T04:20:32.000Z)
2) metrics_facing-stg-v2-datanode-13 (i-64XXXXX, m1.large, 2014-03-04T18:59:26.000Z)
q) Quit

> 2

Logging in as ubuntu to ec2-XXXX.compute-1.amazonaws.com
```