Ym/rrdtool create only

Hello,

This is a double patch on the rrdtool plugin.

1st patch
New config option "CreateRRDOnly".
If set, the plugin will only create a RRD but never update.
This is useful if you want to have one thread to create RRD files (using rrdtool plugin) and another thread to update RRD files (using rrdcached).

2nd patch
New config option "RRA".
Use this option to specify the RRA to use. Default is "AVERAGE,MIN,MAX" (no regression) but you can specify a subset of them.
Useful if you need only AVERAGE : the size of the RRD will be 3 times smaller.

Regards,
Yves