Fix preparation of aki/ari/ami images on Ubuntu

The boto tests currently don't run because euca-bundle-image fails on
Ubuntu; e.g.

---
 ++ euca-bundle-image -r x86_64 \
  -i /opt/stack/new/devstack/files/images/cirros-0.3.2-x86_64-uec/cirros-0.3.2-x86_64-vmlinuz \
  --kernel true \
  -d /opt/stack/new/devstack/files/images/s3-materials/cirros-0.3.2

 euca-bundle-image: error: No such file or directory: /opt/stack/new/devstack/files/images/s3-materials/cirros-0.3.2
---

I noticed this in debugging why Fedora tests are running slower.  I
think that the version of euca-bundle-image in Fedora must create the
directory if it doesn't exist, while the Ubuntu one fails.

I also think this doesn't get caught by errexit because it's running
in a special sub-shell to use other credentials.

Change-Id: I79125e956088353aa76a3f7d6d81af1e4f6a27d4