Automatically collect logs from an AppScale deployment

Adds a tool, `appscale-gather-logs`, that can be used to collect logs from every machine running in an AppScale deployment. Users can specfiy where they want these logs placed, if desired.

Also extends the existing tools to automatically collect logs in case of failures. Indicates to users where the logs have been placed but does not automatically send them to an AppScale server for debugging (an investigation into this will be performed at a later time).