Added power_cycle_nodes script

Simple script that takes a project name and calls the CLI command to power cycle all of its nodes.

This PR was inteded to address the Trello backlog task ```trivial head node script to talk to HaaS API, power cycle machines...```, although now that I read it again I'm not sure that I actually addressed it.  Were we looking for something to actually run *on* the headnode?

I had to do some funky stuff to use the CLI module to do this.  Although, I don't know if there is a better option.
We're using StringIO (not cStringIO) elsewhere in the codebase, maybe we should convert everything over? I don't see an obviouis reason not to use cStringIO everywhere. If people agree, I'll open a separate issue.