Handles new collectd plugins

* disk is renamed to df, and handles complex df (collectd 5)
* disk plugin
* interface handles the collectd 5 plugin
* nginx plugin
Dont override df metrics with df_complex ones
New collectd plugins and some fixes

* New plugins : openvpn, processes, swap, uptime, users
* Dont display nginx active connections, as it is the sum of all the others
Merge branch 'master' into new_definitions
Update collectd definitions with new DSL syntax
Merge pull request #13 from renchap/new_definitions

Handles new collectd plugins