dockertest.images: Fix repo_split_p regexp

Docker looks for either '.' or ':' in order to distinguish between
repo_addr or user name, thus the name could contain other characters
and not just \w.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
Merge pull request #2 from ldoktor/framework

dockertest.images: Fix repo_split_p regexp