add Cluster and Host into csv export

[koebi001]

export data from all hosts for aggregate graphs page

[koebi001]

Without this patch only data from $output_array[0] get exported without further information.
My patch will change that by adding the two columns "Cluster" and "Host". If there are data from multiple hosts like at aggregate graphs the csv gets a line for each time for each host.
Using the csv export if there are multiple host included will probably need some kind of post filtering but at least contains all data at this way...

[vvuksan]

How about adding host cluster in the header ie.

timestamp, disk-free on web01@production, disk_free on web02@production

?

[koebi001]

I changed the code to add host and cluster into the header...