Add image already exists option to boot-seed-vm
Add image already exists option to boot-seed-vm

Add an option to indicate that the image to be used does not need
to be built at this stage.

This differs from a cached image that lives on the disk during
this step.

Change-Id: I0672e2ae20e81b9edef328cc6f71a2a32e8b4707

rdo-management/tripleo,rdo-management/tripleo