Update stage 3 base image

While the base image can be set via the Jenkins job parameters,
the code to use that parameter was not present in the v10 branch.
This change uses the OPENSTACK_BASE_IMAGE parameter

Change-Id: I229cdca38e605b419a7528031fec542049311c48
Signed-off-by: beierlm <mark.beierl@canonical.com>
1 file changed