Add .concourse prefix to concourse

Because bosh-init currently uses template name as job name, it renders
collectd config to say concourse host is called `collectd`. Add a prefix
that explicitly contains `.concourse.` so that we can see which host
are metrics coming from. For more derails see

cloudfoundry-attic/bosh-init#87

Author: mtekel

manifests/concourse-manifest/concourse-base.yml

@@ -152,7 +152,7 @@ jobs:
              # separately, and therefore this value cannot be templated in.
              Host "10.0.16.20"
              Port "2003"
-             Prefix "collectd."
+             Prefix "collectd.concourse."
              EscapeCharacter "_"
              SeparateInstances true
              StoreRates false