TF-A: Migrate nodes to bionic

This change moves us away from the older xenial image, to a newer bionic
image. Also, xenial is running inside ECS, whilst bionic is running in
YADP. So this moves TF-A over to YADP.

Signed-off-by: Benjamin Copeland <ben.copeland@linaro.org>
Change-Id: I1dee7209453e4322f5540b081e3a697d7d0e6d51
diff --git a/tf-a-builder.yaml b/tf-a-builder.yaml
index a42ea88..55453cc 100644
--- a/tf-a-builder.yaml
+++ b/tf-a-builder.yaml
@@ -62,7 +62,7 @@
             name: GERRIT_REFSPEC
             default: 'refs/heads/master'
     disabled: false
-    node: docker-amd64-xenial
+    node: docker-amd64-bionic
     display-name: 'Trusted Firmware A (TF-A) builder'
     scm:
         - tf-a-ci-scripts