Attempt to migrate to xenial images on nodepool

Since march devstack can not be deployed using usual
devstack installation scripts on trusty. We need to mitigate it.
So, the new xenial image already present on CI lab and we need to
update nodepool settings to make workers become updated.
This patch make nodepool use new base image with xenial.

Change-Id: If9a993043c9796b24090bbc07c9a029748fb3189

murano-ci/nodepool/nodepool.yaml

@@ -42,6 +42,7 @@ providers:
         min-ram: 2048
         private-key: '/home/nodepool/.ssh/id_rsa'
         setup: prepare_node_murano_devstack.sh
+
   - name: cilab-2
     username: 'ci-user'
     password: 'swordfish'
@@ -55,11 +56,11 @@ providers:
       - net-id: 'CILAB-2_PRIVATE_NET_ID'
 
     images:
-      - name: trusty-devstack-cz5680
-        base-image: 'trusty-server-cloudimg-amd64-disk1'
+      - name: xenial-devstack-cz5680
+        base-image: 'xenial-server-cloudimg-amd64-disk1'
         min-ram: 2048
         private-key: '/home/nodepool/.ssh/id_rsa'
         setup: prepare_node_murano_devstack.sh
 
 targets:
-  - name: murano-gate
+  - name: murano-gate