Use localhost for mysql collectd plugin

Using {{undercloud_ctlplane_ip_address.stdout}} leads to something like
192_168_24_1 and a new namespace within graphite.

Change-Id: Ic5cf9a181d87a72ca58c5fe0ce01b99936736062

ansible/install/roles/collectd/templates/undercloud.collectd.conf.j2

@@ -190,7 +190,7 @@ PreCacheChain "PreCache"
 {% endif %}
 <Plugin mysql>
   <Database "undercloud">
-    Host  "{{undercloud_ctlplane_ip_address.stdout}}"
+    Host  "localhost"
     User "root"
     Password "{{undercloud_mysql_password.stdout}}"
     InnodbStats true