[WIP][sensu-custom] Modify the disk-usage collection interval from 3600 to 900 sec.

Noticed that the disk usage of `/var/www/proxy/blog_cache` exceeds 100%.
We want to detect this exceeding more earlier,
so modify the disk-usage collection interval.
[sensu-custom] Modify the collection interval of disk-usage.

```
diff --git a/site-cookbooks/sensu-custom/recipes/server_checks.rb b/site-cookbooks/sensu-custom/recipes/server_checks.rb
index d79c76f..9f11a62 100644
--- a/site-cookbooks/sensu-custom/recipes/server_checks.rb
+++ b/site-cookbooks/sensu-custom/recipes/server_checks.rb
@@ -67,7 +67,7 @@ sensu_check "disk-usage-metrics" do
   command "disk-usage-metrics.rb --scheme host:`uname -n`.disk_usage -f"
   handlers ["growthforecast"]
   subscribers ["all"]
-  interval 3600
+  interval 900
 end

 sensu_check "load-metrics" do
```
Merge pull request #164 from kazu634/disk-usage-collection-interval

[sensu-custom] Modify the disk-usage collection interval from 3600 to 900 sec.