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.
I think we shouldn't be importing haas.cli from higher-level scripts; it would be cleaner to just invoke the cli command as a subprocess -- it really isn't designed to be used as a library. Really, we should have a proper client library.