Move salt formulas test to xenial jenkins node

Currently some tests are not build-able on jenkins bionic nodes

Related-bug: PROD-36062
Change-Id: I99990428ae976f2e8ee9d9d091c1a5fb7f0796c4
diff --git a/test-salt-formulas-pipeline.groovy b/test-salt-formulas-pipeline.groovy
index e537ee1..09a7f6c 100644
--- a/test-salt-formulas-pipeline.groovy
+++ b/test-salt-formulas-pipeline.groovy
@@ -13,7 +13,7 @@
 def gerritRef = env.GERRIT_REFSPEC ?: null
 def defaultGitRef = env.DEFAULT_GIT_REF ?: null
 def defaultGitUrl = env.DEFAULT_GIT_URL ?: null
-def slaveNode = env.SLAVE_NODE ?: 'python&&docker'
+def slaveNode = env.SLAVE_NODE ?: 'old16.04'
 def saltVersion = env.SALT_VERSION ?: ""
 def dockerLib = new com.mirantis.mk.Docker()