Adding poweroff command for use in noop model/policy

This pull request adds a `poweroff` command to the list of commands supported by the Microkernel Controller. This additional command is needed in order to support the commands returned to the Microkernel after it checks into a node that has been bound to the new `discovery_only` noop model that was recently added to Hanlon. Once we're sure this all works, the Hanlon and Hanlon-Microkernel projects will both be tagged and new releases of both projects will be constructed to enable these new noop models/policies.