utilize DISTRO default from build-unit-test-docker

Both run-unit-test-docker.sh and build-unit-test-docker have a default
for DISTRO. We don't want users to have to update both when
changing the default use the common denominator which is
build-unit-test-docker.

Change-Id: Id2813570d1104c97ee6d8737960a103c1e028b52
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
1 file changed