Doc glance hw_ namespace for hardware properties

fixes bug 1153470

As noted in the bug report, glance hardware properties now use
'hw_' as a namespace. There was only one reference to this in
the docs, which this patch updates.

Change-Id: I7af8f5f602c8b7752acd50c7b152a1dedeb1b615
Merge "Doc glance hw_ namespace for hardware properties"