Slave: Update TF-M jobs to use Bionic over Xenial

Signed-off-by: Karl Zhang <karl.zhang@arm.com>
Change-Id: Icf49df65bc80697b5e02aef1d18e863ff5187127
diff --git a/jenkins/ci.jpl b/jenkins/ci.jpl
index cb5f990..28671a1 100644
--- a/jenkins/ci.jpl
+++ b/jenkins/ci.jpl
@@ -146,7 +146,7 @@
 def builds = [:]
 def results = [:]
 
-node("docker-amd64-xenial") {
+node("docker-amd64-bionic") {
   stage("Init") {
     cleanWs()
     dir("tf-m-ci-scripts") {
@@ -164,6 +164,7 @@
     builds["docs"] = buildDocs(results)
   }
 }
+
 stage("Builds") {
   def verify = 1
   try {
@@ -182,7 +183,7 @@
   }
 }
 
-node("docker-amd64-xenial") {
+node("docker-amd64-bionic") {
   stage("Copy Docs") {
     step([$class: 'CopyArtifact', projectName: 'tf-m-build-docs',
     selector: specific("${results['docs'][0]}"), target: './docs/',