Fix to allow collectd.prefix on 14.04 and non-12.04 Ubuntu.

Add missing default from map.jinja to give a default value on Trusty.
Merge pull request #33 from ministryofjustice/collectd_prefix_trusty_fix

Fix to allow collectd.prefix on 14.04 and non-12.04 Ubuntu.
Merge branch 'master' into grafana_scripts

Conflicts:
	CHANGELOG.md