Accept None as value of slave.user

Change-Id: I79ccdb44d85b803c29c6b1ca157862449e480593
diff --git a/jenkins/files/slave/default b/jenkins/files/slave/default
index 7bfbade..cbd0065 100644
--- a/jenkins/files/slave/default
+++ b/jenkins/files/slave/default
@@ -26,7 +26,7 @@
 # jenkins /run location
 JENKINS_RUN=/var/run/jenkins
 
-{%- if slave.user is defined %}
+{%- if slave.get('user', None) %}
 # Authentication to jenkins master
 JENKINS_LOGIN="{{ slave.user.name }}"
 JENKINS_PASSWORD="{{ slave.user.password }}"