dockertest: Modify {containers,images).get_unique_name()

Make {containers,images).get_unique_name() always prepend the test name
and remove all unnecessarily prefixes from tests. The sensible ones were
kept (eg. server, client), but still the test name is going to be
prepended automatically.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
Merge pull request #357 from ldoktor/name_prefix

dockertest: Modify {containers,images).get_unique_name()