Refactor use of docker images in mk/Debian.groovy

Use docker image from tcpcloud on dockerhub if possible

Change-Id: Ibf17ee366a4236e12e1fe6120088b0d2907f4bbc
diff --git a/src/com/mirantis/mk/Docker.groovy b/src/com/mirantis/mk/Docker.groovy
index 1641cbd..1357dc1 100644
--- a/src/com/mirantis/mk/Docker.groovy
+++ b/src/com/mirantis/mk/Docker.groovy
@@ -30,3 +30,25 @@
         params.join(' ')
     )
 }
+
+/**
+ * Build step to build docker image.
+ *
+ * @param dockerHubImg     Name of image on dockerhub (ie: tcpcloud/salt-models-testing)
+ * @param defaultImg       Image to use if dockerHubImg is not found
+ * @return img             Docker image
+ */
+
+def getImage(dockerHubImg, defaultImg="ubuntu:latest") {
+
+    def img
+
+    try {
+        img = docker.image(dockerHubImg)
+        img.pull()
+    } catch (Throwable e) {
+        img = docker.image(defaultImg)
+    }
+
+    return img
+}
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