[Brightbox] Updates image selector for name format

Brightbox is now moving to using the default Ubuntu Cloud Images
(http://cloud-images.ubuntu.com/) where we are also using the naming
convention used there.

So the image selector has been updated to select the latest version of
Ubuntu (i386, official Brightbox) image if no image is specified with
the new format.
Merge pull request #1793 from fog/brightbox_image_selector

[Brightbox] Updates image selector for name format