dockertest.images: Switch to TextTable

Switch to using output.TextTable to parse
docker images output.  This is for consistency
with containers module and to help verify
header-column widths always matches content widths.

Also added ``images_args`` attribute in case
instances need to pass more/different arguments
than just ``--no-trunc``.  Removed a no-longer
relevant FIXME.

Signed-off-by: Chris Evich <cevich@redhat.com>
Merge pull request #193 from cevich/TextTable_images

dockertest.images: Switch to TextTable