[magnum] Fix dib elements path
[magnum] Fix dib elements path

The diskimage-builder elements location has changed [1].

[1] http://git.openstack.org/cgit/openstack/diskimage-builder/tree/diskimage_builder/elements

Change-Id: I4f830188460d188fd681941d528ef97c8c0aaea3

openstack-infra/project-config,openstack-infra/project-config