adds support for write_riemann plugin

With this PR a new salt state is introduced: `collectd.write_riemann`. This requires a recent version of collectd package (>= 5.3), which is available in Ubuntu 14.04, Debian Jessie and RedHat 7.

As the `collectd-core` package does not (at least for Ubuntu) include the needed protobuf-c.h files (see Dependencies in https://collectd.org/wiki/index.php/Plugin:Write_Riemann), the required package is now the meta package `collectd`.