Drop default 'ubuntu' user

Change-Id: Idf49bf07c9df07ee84fa2ad8b1c9cc8310d52671
Closes-Bug: PROD-23708 (PROD:23708)
diff --git a/common/ubuntu_security.sh b/common/ubuntu_security.sh
index ab54be9..8d7c8c1 100644
--- a/common/ubuntu_security.sh
+++ b/common/ubuntu_security.sh
@@ -51,6 +51,9 @@
 # Disable password root login
 usermod -p '!' root
 
+# Drop default 'ubuntu' user
+userdel -rf ubuntu
+
 # Disable SSH password authentication and permit root login
 sed -i 's|[#]*PasswordAuthentication yes|PasswordAuthentication no|g' /etc/ssh/sshd_config
 sed -i 's|[#]*PermitRootLogin.*|PermitRootLogin no|g' /etc/ssh/sshd_config