Allow the usage of shortname for client's name

Currently we force the usage of the FQDN as the name of the client.
In some case, for example if the sensu client is used as a relay for
another monitoring system, we want to have the choice to use the hostname.

Change-Id: I67be453213b21cdc2c7f05fa896586e71fef933b
diff --git a/sensu/files/client.json b/sensu/files/client.json
index 9f5955d..37a01c6 100644
--- a/sensu/files/client.json
+++ b/sensu/files/client.json
@@ -3,7 +3,11 @@
 {%- from "sensu/map.jinja" import client with context -%}
 {
   "client": {
+    {%- if client.get('use_shortname', False) %}
+    "name": "{{ system.name }}",
+    {%- else %}
     "name": "{{ system.name }}.{{ system.domain }}",
+    {%- endif %}
     "hostname": "{{ system.name }}",
     "fqdn": "{{ system.name }}.{{ system.domain }}",
     "cluster": "{{ pillar.linux.system.cluster }}",