added empty config.json

create config.json file, for example salesforce plugin is expecting this file and is failing without it

Change-Id: Ie68cc217b0013251d6e63bd8d768e69f3b7dff99
diff --git a/sensu/server.sls b/sensu/server.sls
index 9162950..7d0207c 100644
--- a/sensu/server.sls
+++ b/sensu/server.sls
@@ -17,6 +17,11 @@
   - require:
     - pkg: sensu_server_packages
 
+/etc/sensu/config.json:
+  file.managed:
+  - contents: "{}"
+  - replace: false
+
 {%- if server.mine_checks %}
 
 {%- for node_name, node_grains in salt['mine.get']('*', 'grains.items').iteritems() %}