Fix DuplicateOptError in fixed_key migration code
Fix DuplicateOptError in fixed_key migration code

Fix how the key migration code loads the fixed_key config option so that
it handles DuplicateOptError.

Closes-Bug: #1747021
Change-Id: I119a12c8e80ad991f5a2916fec2ef4421039b9e9

j-griffith/cinder,mahak/cinder,phenoxim/cinder,j-griffith/cinder,Datera/cinder,openstack/cinder,mahak/cinder,openstack/cinder,Datera/cinder,phenoxim/cinder