Build to a disabled host via the API

This functionality currently exists but only when using availability zones. A method should exist for building to a disabled host via the API even if AZ's are not in use.

This would need to be a small change to the compute_filter to not fail out a host if it is disabled but specified as a targeted host. See related blueprint: https://blueprints.launchpad.net/nova/+spec/scheduler-hints-for-cell-and-host-override which covers the addition of a filter for targeting builds to cells/hosts via the API.

This can be wrapped around the same security context, force_host, that the AZ feature currently uses.