Change CI jobs to run on standard 8GB RAM nodes

Partially revert I5f8dc972f67c5649bf5f9403a5a512d06c948720.

Change-Id: I81e393c78a50ba9c1149d214f07a1a2dc8524b82

.zuul.yaml

@@ -47,7 +47,6 @@
 
 - job:
     name: airship-images-build
-    nodeset: airship-images-single-node-32GB
     timeout: 7200
     post-timeout: 7200
     pre-run: playbooks/airship-images-deploy-docker.yaml
@@ -56,7 +55,6 @@
 
 - job:
     name: airship-images-publish
-    nodeset: airship-images-single-node-32GB
     timeout: 7200
     post-timeout: 7200
     pre-run: playbooks/airship-images-deploy-docker.yaml
@@ -82,12 +80,6 @@
       - name: primary
         label: ubuntu-bionic
 
-- nodeset:
-    name: airship-images-single-node-32GB
-    nodes:
-      - name: primary
-        label: ubuntu-bionic-32GB
-
 - secret:
     name: images_airshipit_github_secret
     data: