Log multiple different websites

This requires some thought.

It should be easy to require the slave sites to use a site-specific key for their memcache data or a different bin or a different server.
I think we should only support "bin" and "server".

For the graphical representation, we need to introduce another level.

This is now implemented and should work. Needs testing though.

works now, needs docs

$conf['rrdtool_logged_servers'] = array(
'localhorst' => array(
'memcache_bins' => array('rrdtool' => 'default'),
'memcache_servers' => array(
'127.0.0.1:11211' => 'default',
),
),
);

Docs are essential before we role an official release, but dropping priority for the time being.

Docs issue is #855272: Update and extend docs