Enforced nightly version of salt-models-testing docker image during salt models test

Change-Id: Ida5b159df700be8caf4e2fc1522f2e2e9ae25f0a
diff --git a/src/com/mirantis/mk/SaltModelTesting.groovy b/src/com/mirantis/mk/SaltModelTesting.groovy
index 6aec0c3..8bb643f 100644
--- a/src/com/mirantis/mk/SaltModelTesting.groovy
+++ b/src/com/mirantis/mk/SaltModelTesting.groovy
@@ -24,7 +24,7 @@
   def saltOpts = "--retcode-passthrough --force-color"
   def common = new com.mirantis.mk.Common()
   def workspace = common.getWorkspace()
-  def img = docker.image("tcpcloud/salt-models-testing:${formulasRevision}")
+  def img = docker.image("tcpcloud/salt-models-testing:nightly")
   img.pull()
 
   if (!extraFormulas || extraFormulas == "") {