Stop creating disk images during wipe-openstack.

Our principled separation of concern places one-time setup like flavor
creation and baremetal deploy ramdisk creation and registration
outside the boot-stack problem-space. The incubator devtest story now
does that, so doing it here is just redundant.

Change-Id: I63cf52b6dca48f8f75b550388c6912eda5c0f533