Nodepool: Use the default security group

Use the built-in "default" security group for allowing egress traffic
instead of "pp-net"

Change-Id: I52721a754e4e26db4478c7d3dc961e28625b3516
diff --git a/nodepool/nodepool.yaml b/nodepool/nodepool.yaml
index d0cb9e1..f8a7122 100644
--- a/nodepool/nodepool.yaml
+++ b/nodepool/nodepool.yaml
@@ -62,7 +62,7 @@
           - lab-vlan400
           - lab-charmed-testing-iscsi
         security-groups:
-          - pp-net
+          - default
           - "Remote Access"
         labels:
           - name: centos-base
@@ -74,7 +74,7 @@
         max-servers: 4
         auto-floating-ip: False
         security-groups:
-          - pp-net
+          - default
           - "Remote Access"
         networks:
           - lab-charmed-testing-ssh
@@ -88,7 +88,7 @@
         max-servers: 1
         auto-floating-ip: False
         security-groups:
-          - pp-net
+          - default
           - "Remote Access"
         networks:
           - lab-charmed-testing-ssh