Move Ubuntu builds away from nb04

Something about the debootstrap processes destroys the nodepool
container, making not only this Ubuntu builds fail but then every
build after.

Move them back to regular builders while we debug further.

Change-Id: I48eb721bffafe2404a332192976848dba2ca80cc

nodepool/nb04.opendev.org.yaml

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

nodepool/nodepool.yaml

@@ -329,13 +329,29 @@ diskimages:
 
   ############ Plain images #############
 
-  # NOTE : these only build on nb04; see that configuration file
-
   - name: ubuntu-bionic-plain
-    pause: true
+    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'
 
   - name: ubuntu-xenial-plain
-    pause: true
+    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
 
   - name: opensuse-15-plain
     pause: true