Highlight use of 'file' Glance backend

Highlight the use of the 'file' Glance backend for storage and the
default location that images will be stored under this scheme. Also
provide some guidance on the amount of space this will require.

Change-Id: I412c938d5f2151b3f3f76d47542d25d67c1c9869
Closes-bug: #1239516
backport: havana
(cherry picked from commit eec6cdb7be32799e3a56fd5d7000d4c0435e7158)