Ensure install of diskimage-create.sh requirements
Ensure install of diskimage-create.sh requirements

Change I612ea1c583090897bd44453b867d75929a01b7fc [1] removed
diskimage-builder from the root requirements.txt file. Devstack
deployments not setting LIBS_FROM_GIT+=diskimage-builder will not
install diskimage-builder. This means our diskimage-create.sh tool will
fail to build amphora images and abort the deployment.

[1] https://review.opendev.org/#/c/741960/

Change-Id: Id535d2d5ea6c23c9646c568a17d7695f82c0c1a9

openstack/octavia,openstack/octavia,openstack/octavia