Reimplement reserve_nodes and release_nodes.

Implement these two methods based on a host, rather than based on
an instance_uuid.

Also adds some doc strings to db/api.

Part of bp:ironic-manager

Change-Id: I41ddcec90f4dc2b22712cdb069a3d4626fb7c19d
Merge "Reimplement reserve_nodes and release_nodes."