Add support for testing of local SDC images

Add support for testing of local SDC images

By default it is disabled and the test works as before, but by providing
the SDC_LOCAL_IMAGES variable with a value of true/yes it will not use
the upstream images (from registry) but instead the local one (in docker
cache) - they have to be build beforehand of course.

They are two more optional variables (if SDC_LOCAL_IMAGES == true/yes):
SDC_LOCAL_GITREPO .. for local git repo instead of an upstream branch
SDC_LOCAL_TAG     .. it will use local images with this docker tag

Issue-ID: SDC-2553

Signed-off-by: Petr Ospalý <p.ospaly@partner.samsung.com>
Change-Id: Ib64ca17e81aa9c3406e0a5d8fb403f1c1b0862a1