Minor rosctl updates

A few almost trivial things to get it to work for me:

**Notifications**

There is no NOTIFY_USERS on my machine (trusty) or precise, where does this come from? Instead I just enabled it so it checks for available notify-send or not.

**ROS_HOSTNAME**

This will usually get set by the setup script. Check for its existence before setting it.

**Temporary Log File**

You'll get permission problems if multiple people are running rosctl and for whatever reason, the log file isn't cleaned up. I'm adding a timestamp to it here to uniquify it.