Fix Disk image builder steps for creating magnum images

[yatin]

Current steps are not working due to inexistence of diskimage-builder/bin/disk-image-create

This needs to be fixed.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/449063

[Spyros Trigazis (strigazi)]

Pointers to the change:
http://git.openstack.org/cgit/openstack/diskimage-builder/commit/?id=7446c32197e01407a7df703e28efbdeda282a03d

And we also need the same fix in ci:
http://git.openstack.org/cgit/openstack-infra/project-config/tree/jenkins/jobs/magnum.yaml#n219

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/449063
Committed: https://git.openstack.org/cgit/openstack/magnum/commit/?id=d93a813c391e9bcdeb04c133f5b7ccb44088b266
Submitter: Jenkins
Branch: master

commit d93a813c391e9bcdeb04c133f5b7ccb44088b266
Author: yatinkarel <email address hidden>
Date: Thu Mar 23 16:18:13 2017 +0530

    Update Steps for creating dib images

    diskimage-builder/bin/disk-image-create no longer exists.
    So update steps for creating disk images.

    Change-Id: Ieae16ac32641f7525feb6caa0c55caa29615df9e
    Closes-Bug: #1675285

[OpenStack Infra (hudson-openstack)]

This issue was fixed in the openstack/magnum 5.0.0 release.