Merge branch 'change_diskimage_to_xenial' into 'master'

Update diskimages to be ubuntu-xenial

See merge request !9

nodepool/nodepool.yaml

@@ -51,7 +51,7 @@ diskimages:
         - growroot
         - infra-package-needs
         - stackviz
-      release: trusty
+      release: xenial
       env-vars:
         TMPDIR: /opt/dib_tmp
         DIB_IMAGE_CACHE: /opt/dib_cache