nodepool: m1.xlarge for StorPool, username: ubuntu for the charm one

The default Ubuntu cloud image does not have a "zuul" user account, does it now?
And 8GB do not seem to be enough for the default StorPool cgroups configuration.

Change-Id: I428eea8dc6d62d0e05f01f20153ccaf8da38bd9f
diff --git a/nodepool/nodepool.yaml b/nodepool/nodepool.yaml
index 9a44c22..9463b4b 100644
--- a/nodepool/nodepool.yaml
+++ b/nodepool/nodepool.yaml
@@ -51,6 +51,7 @@
     cloud-images:
       - name: pp-base-01
       - name: ubuntu-focal
+        username: ubuntu
     pools:
       - name: main
         max-servers: 3
@@ -64,7 +65,7 @@
         labels:
           - name: centos-base
             cloud-image: pp-base-01
-            flavor-name: m1.large
+            flavor-name: m1.xlarge
             boot-from-volume: True
             volume-size: 80
       - name: zaza-main