replace bionic with xenial image as default

Change-Id: I0ac4e3551c619f08ba0695ec605383ddc3182a20
diff --git a/config-drive/master_config.sh b/config-drive/master_config.sh
index 548c390..485d1df 100644
--- a/config-drive/master_config.sh
+++ b/config-drive/master_config.sh
@@ -57,6 +57,9 @@
     maas ${PROFILE} boot-source delete 1
     maas ${PROFILE} boot-resources import
     # TODO wait for finish,and stop import.
+  else
+    maas ${PROFILE} boot-source-selections create 1 os="ubuntu" release="xenial" arches="amd64" subarches="*" labels="*"
+    maas ${PROFILE} boot-resources import
   fi
 }