Add a note regarding image properties for volumes

As introduced in Havana, the glance_core_properties option
in cinder is an important consideration if the core properties
of images are changed. This patch adds a note in the image metadata
section.

Change-Id: Icdc04c742349cfa9d6013dcb51b1f904e6fab391
Closes-Bug: 1329419
Merge "Add a note regarding image properties for volumes"