diamond: Explicitly set method to use for determining hostname

Currently, tools-webproxy.eqiad.wmflabs gets reported as
just tools to graphite, causing issues with shinken (and anything
else that assumes name of host is name of metric in graphite).
This explicitly sets the method to use as hostname, so we
consistently get the same results everywhere.

Change-Id: I08927cf0501a45bd9e776aa79e1fc4bd6cf01217