Read default_ephemeral_format from nova.conf

Currently the default_ephemeral_format is coming through as
blank as the existing reference has no parent dict, so is not
being retrieved.

Change-Id: I3553ba27a18ec64a710168f0026fb8dd2aaed45f
Closes-Bug: #1301841
Merge "Read default_ephemeral_format from nova.conf"