CI: Use Ubuntu Bionic for nodeset

Currently, treasuremap CI uses Ubuntu-Xenial. This change moves to
Ubuntu-Bionic in order to use the version of Docker requested by the
OpenStack-Helm minikube script.

Change-Id: Iba9bfda21c5a9995c775ff9bdc6bc9e0e2260d1b

.zuul.yaml

@@ -38,8 +38,8 @@
 - nodeset:
     name: treasuremap-single-node
     nodes:
-      - name: ubuntu-xenial
-        label: ubuntu-xenial
+      - name: ubuntu-bionic
+        label: ubuntu-bionic
 
 - job:
     name: treasuremap-site-lint

tools/gate/playbooks/airskiff-deploy-gate.yaml

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-- hosts: ubuntu-xenial
+- hosts: ubuntu-bionic
   tasks:
     - name: Install required packages
       shell: |

tools/gate/playbooks/airskiff-reduce-site.yaml

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-- hosts: ubuntu-xenial
+- hosts: ubuntu-bionic
   tasks:
     - name: Replace Armada manifest
       shell: |

tools/gate/playbooks/debug-report.yaml

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-- hosts: ubuntu-xenial
+- hosts: ubuntu-bionic
   tasks:
     - name: Build debug report
       shell: |

tools/gate/playbooks/generate-certs.yaml

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-- hosts: ubuntu-xenial
+- hosts: ubuntu-bionic
   tasks:
     - name: Generate site certificates
       shell: |

tools/gate/playbooks/git-config.yaml

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-- hosts: ubuntu-xenial
+- hosts: ubuntu-bionic
   tasks:
     - name: Git config
       shell: |

tools/gate/playbooks/install-docker.yaml

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-- hosts: ubuntu-xenial
+- hosts: ubuntu-bionic
   tasks:
     - name: Install Docker CE
       shell: |

tools/gate/playbooks/site-lint.yaml

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-- hosts: ubuntu-xenial
+- hosts: ubuntu-bionic
   tasks:
     - name: Lint site
       shell: |