Decouple state inspection and availability check

The state of a node should only be considered when calculating
resources, not when determining availability.  This makes the driver
consider any node that exists in the node inventory as available, and
take into account its state when determining available resources. All nodes
will be presented to Nova, but those that are in maintenence mode, have no
power state or have an error power state will present zero resources.

Closes-Bug: #1309048
Change-Id: Ia78294ac5d39955dac99d3d773751e8f7538a74d
Merge "Decouple state inspection and availability check"