Switch the default nodeset to e2

Google has requested that we use e2 nodes, which have shared cpu.

Also add a "-large" nodeset for jobs where we need 16G of ram.

Change-Id: Id42298e229aae413935d8fcc8ba58c4071e5123b

zuul.d/nodesets.yaml

diff --git a/zuul.d/nodesets.yaml b/zuul.d/nodesets.yaml
index 0e11d14..07d9368 100644
--- a/zuul.d/nodesets.yaml
+++ b/zuul.d/nodesets.yaml
@@ -2,4 +2,10 @@
     name: debian-stretch
     nodes:
       - name: testnode
-        label: debian-stretch-8G
+        label: debian-stretch-e2-standard-8
+
+- nodeset:
+    name: debian-stretch-large
+    nodes:
+      - name: testnode
+        label: debian-stretch-e2-standard-16