Revert "Move Ubuntu builds away from nb04"

This reverts commit 03a2a71984f921fdc73466cd7781799d2982369d.

Change I0887a801bb6dd4ce992c39d9e332a18f8194a7b9 merged in nodepool,
which should work around the problem we saw on nb04.  Try again.

Change-Id: I4a13d4f0f07d755a12557b50ec24ba59df51fe97

nodepool/nb04.opendev.org.yaml

@@ -332,7 +332,7 @@ diskimages:
 
   - name: ubuntu-bionic-plain
     parent: base-debuntu
-    pause: true
+    pause: false
     elements:
       - ubuntu-minimal
       - cache-devstack
@@ -343,7 +343,7 @@ diskimages:
 
   - name: ubuntu-xenial-plain
     parent: base-debuntu
-    pause: true
+    pause: false
     elements:
       - ubuntu-minimal
       - cache-devstack

nodepool/nodepool.yaml

@@ -329,29 +329,13 @@ diskimages:
 
   ############ Plain images #############
 
+  # NOTE : these only build on nb04; see that configuration file
+
   - name: ubuntu-bionic-plain
-    parent: base-debuntu
-    pause: false
-    elements:
-      - ubuntu-minimal
-      - cache-devstack
-    release: bionic
-    env-vars:
-      DIB_DISTRIBUTION_MIRROR: 'http://mirror.dfw.rax.openstack.org/ubuntu'
-      DIB_DEBIAN_COMPONENTS: 'main,universe'
+    pause: true
 
   - name: ubuntu-xenial-plain
-    parent: base-debuntu
-    pause: false
-    elements:
-      - ubuntu-minimal
-      - cache-devstack
-    release: xenial
-    env-vars:
-      DIB_DISTRIBUTION_MIRROR: 'http://mirror.dfw.rax.openstack.org/ubuntu'
-      DIB_DEBIAN_COMPONENTS: 'main,universe'
-
-  # NOTE : these only build on nb04; see that configuration file
+    pause: true
 
   - name: opensuse-15-plain
     pause: true