fix(grafana,telegraf): make curl fail on unsuccessful HTTP codes

[mboersma]

Adds the -f flag to curl downloads to avoid the possibility of creating a file with contents like:

$ cat /usr/bin/telegraf 
<?xml version='1.0' encoding='UTF-8'?><Error><Code>AccessDenied</Code><Message>Access denied.</Message><Details>Anonymous users does not have storage.objects.get access to object telegraf/telegraf.</Details></Error>

See #156. Thanks to @felixbuenemann for suggesting the fix!

[deis-bot]

@jchauncey, @helgi and @krancour are potential reviewers of this pull request based on my analysis of git blame information. Thanks @mboersma!