Moved the DockerWorker class from module file into its separate file
in module_utils directory for future extension.
Unit tests changed accordingly.

Signed-off-by: Ivan Halomi <ivan.halomi@tietoevry.com>
Co-authored-by: Martin Hiner <martin.hiner@tietoevry.com>
Change-Id: Ia2a471a9a2805e13b2c20dbf8a7297c23231aae3