Always set user and password in minion config

Even when we don't have auth enabled on Jenkins side, set dummy credentials.
Otherwise jenkins module will complain about missing parameters.

Change-Id: Iad2d1d1551c2cbfc9a0462466bdcef909cdceed8
diff --git a/jenkins/files/_jenkins.conf b/jenkins/files/_jenkins.conf
index c0ac891..0079d74 100644
--- a/jenkins/files/_jenkins.conf
+++ b/jenkins/files/_jenkins.conf
@@ -3,7 +3,7 @@
   url: "{{ client.master.get('proto', 'http') }}://{{ client.master.host }}:{{ client.master.port }}"
   {%- if client.master.api_key is defined %}
   api_key: {{ client.master.api_key }}
-  {%- elif client.master.get('username', None) %}
-  user: {{ client.master.username }}
-  password: {{ client.master.password }}
+  {%- else %}
+  user: {{ client.master.get('username', 'admin') }}
+  password: {{ client.master.get('password', 'dummy') }}
   {%- endif %}