Fix race condition while creating dest dir for docker images

Fix race condition while creating dest dir for docker images

Docker_downloader module uses threads to concurrently save
docker images to dest dir which creation is not guarded with any
kind of thread lock object thus it could fail on os.makedirs as other
thread could have already created that dir. Hence "exist_ok=True"
opt is added to os.makedirs call so that it does not fail in such
circumstances.

Change-Id: I6e2d2c9864b71d038e7b2ed3018cdd3c01916956
Issue-ID: OOM-2631
Signed-off-by: Bartek Grzybowski <b.grzybowski@partner.samsung.com>