Parameterize disk-image-create
Parameterize disk-image-create

The distro and output name were hard-coded in the disk-image-create
call, which made it awkward to run on rhel7.

eprasad/instack-undercloud,bcrochet/instack-undercloud,dpkshetty/instack-undercloud,rdo-management/instack-undercloud,agroup/instack-undercloud,dmsimard/instack-undercloud,dpkshetty/instack-undercloud,eprasad/instack-undercloud,dmsimard/instack-undercloud,agroup/instack-undercloud,bcrochet/instack-undercloud,rdo-management/instack-undercloud