run-unit-test-docker.sh: Don't force ubuntu:latest

Allow ubuntu docker images other than the 'latest' tag to run. 'latest'
points to Ubuntu 16.04 on Docker Hub[0], which isn't really what we're
after.

This leaves in place forcing Fedora to be Ubuntu, but that should
probably change as well.

[0] https://hub.docker.com/_/ubuntu/

Tested: Ran `DISTRO=ubuntu:artful ./run-unit-test-docker.sh` and
verified the environment was Artful.

Change-Id: Ibdcab220db7388e0f1153337cbde01fc4b898883
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
diff --git a/run-unit-test-docker.sh b/run-unit-test-docker.sh
index 0a2ed9c..659c4ab 100755
--- a/run-unit-test-docker.sh
+++ b/run-unit-test-docker.sh
@@ -23,10 +23,7 @@
 # Timestamp for job
 echo "Unit test build started, $(date)"
 
-# Currently only support ubuntu:latest due to systemd requirements
-if [[ "${DISTRO}" == "ubuntu"* ]]; then
-    DISTRO="ubuntu:latest"
-elif [[ "${DISTRO}" == "fedora" ]]; then
+if [[ "${DISTRO}" == "fedora" ]]; then
     echo "Distro (${DISTRO}) not supported, running as ubuntu"
     DISTRO="ubuntu:latest"
 fi