tests: factorize docker tests using docker_exec_cmd logic

tests: factorize docker tests using docker_exec_cmd logic

avoid duplicating test unnecessarily just because of docker exec syntax.
Using the same logic than in the playbook with `docker_exec_cmd` allow us
to execute the same test on both containerized and non containerized environment.

The idea is to set a variable `docker_exec_cmd` with the
'docker exec <container-name>' string when containerized and
set it to '' when non containerized.

Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>