Fixes #174 (Add noop models to Hanlon)

This pull request adds two new models (a `discover_only` model and a `boot_local` model) to Hanlon along with the associated `discover_only` and `boot_local` policies). With these two models in place, new nodes (with no operating system on them) or existing nodes (that already have an operating system installed on them) can be added to the Hanlon inventory without provisioning an OS instance to those nodes (or handing them off to a broker). To make the `discover_only` model work properly, however, an associated PR in the Hanlon-Microkernel was required (to add support for a `poweroff` command). A pre-built version of this modified Microkernel has been uploaded [here](https://www.dropbox.com/s/xcoilu3h69g32g5/hnl_mk_debug-image.1.1.0%2B1-g7e70a8a.iso?dl=0), although a new release of both Hanlon and the Hanlon-Microkernel will be built once this is generated.