Add an api call to power cycle a node.

This involved a handful of changes:

1. Add three columns to the Node class, with ipmi-related info.
2. Add corresponding parameters to Node.__init__ and node_register.
3. Add the api call itself, node_power_cycle.
4. Add the cli wrapper.
5. Update some of the tests due to (2).
Merge pull request #206 from zenhack/powercycle-proxy

Add an api call to power cycle a node.