Force ReportReserved to true in df plugin

It's the Collectd 5 default and only possible setting
Merge pull request #119 from jpoittevin/df-parameter

Force ReportReserved to true in df plugin
Handle plugin order with collectd::plugin resource type
Refactor amqp plugin using collectd::plugin resource type
Refactor apache plugin using collectd::plugin resource type
Refactor bind plugin using collectd::plugin resource type
Refactor csv plugin using collectd::plugin resource type
Refactor df plugin using collectd::plugin resource type
Refactor disk plugin using collectd::plugin resource type
Refactor entropy plugin using collectd::plugin resource type
Refactor filecount plugin using collectd::plugin resource type
Refactor load plugin using collectd::plugin resource type
Refactor interface plugin using collectd::plugin resource type
Refactor users plugin using collectd::plugin resource type
Refactor memcached plugin using collectd::plugin resource type
Refactor memory plugin using collectd::plugin resource type
Refactor ntpd plugin using collectd::plugin resource type
Refactor openvpn plugin using collectd::plugin resource type
Refactor redis plugin using collectd::plugin resource type
Refactor syslog plugin using collectd::plugin resource type