Fix selection of image(s) tested by tempest.

The variable DEFAULT_IMAGE_NAME is set to 'cirros-0.3.0-x86_64-uec' by default.
This will cause configure_tempest to 'exit 1' and abort stack.sh if an image
with that name is not uploaded to glance. According to the relevant code
comment, this behaviour is incorrect. Updated code to match behaviour described
in comment: If image with name matching DEFAULT_IMAGE_NAME exists, use it for
both primary and secondary test image otherwise select first image and, if
available, second image listed by glance. Will still 'exit 1' if no images
are available at all (though it probably shouldn't).

Change-Id: I92773d4afd52cf533d16772ae2a087e23e206f8c
Fixes: bug #1092713
Merge "Fix selection of image(s) tested by tempest."