In a previous patch, the tobiko roles where modified to download Ubuntu
images prior to test execution
With this patch, the Ubuntu tests stop downloading images and use the
previously downloaded images instead
Depends-On: I1dbf739850bfc3f963a789db0dcd2625ffe4a16f
Change-Id: I8d0bc73bf1bb2e031a61b39d24fc1442978f07bd