Improve OS::Compute::HostCapabilities description

The OS::Compute::HostCapabilities namespace should include more
information about how the properties are used by Nova.
Let admin's know that the ComputeCapabilitiesFilter needs to
be enabled.

Change-Id: Ib3232e560969462d3973606e138a96d1a5beb7de
Closes-bug: 1367860
Merge "Improve OS::Compute::HostCapabilities description"