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/lava-submit.jpl b/jenkins/lava-submit.jpl
index 4be24b5..f247953 100644
--- a/jenkins/lava-submit.jpl
+++ b/jenkins/lava-submit.jpl
@@ -6,7 +6,7 @@
 //
 //-------------------------------------------------------------------------------
 
-node("docker-amd64-xenial") {
+node("docker-amd64-bionic") {
   stage("Init") {
     cleanWs()
     dir("tf-m-ci-scripts") {