Unpause Xenial builds

dib 2.14.1 should address the issues we had with getting a network on
boot (Iae88a3b0457bab0b8f0fd1febf58732ca95e5dc9).  Note that building
this will release pip10 more widely, as it will be included in the
base image.

Change-Id: I5b64e4dbd5693ce63520c72e6033c7242eba8deb

nodepool/nodepool.yaml

@@ -301,7 +301,7 @@ diskimages:
       GIT_HTTP_LOW_SPEED_TIME: '300'
       GIT_HTTP_LOW_SPEED_LIMIT: '1000'
   - name: ubuntu-xenial
-    pause: true
+    pause: false
     elements:
       - ubuntu-minimal
       - vm