Update label for salt-formula tests

Jenkins nodes labeled "old16.04" were deleted

Change-Id: If57a195688f1abeb65858cb5527bc57fbde18f40
diff --git a/test-salt-formulas-pipeline.groovy b/test-salt-formulas-pipeline.groovy
index 6cb3b30..958cd1e 100644
--- a/test-salt-formulas-pipeline.groovy
+++ b/test-salt-formulas-pipeline.groovy
@@ -14,7 +14,7 @@
 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 ?: 'old16.04'
+def slaveNode = env.SLAVE_NODE ?: 'docker'
 def saltVersion = env.SALT_VERSION ?: ""
 
 gerritBranch = 'master'