GraphiteWriter: Add support for customized metric prefix names

Allows to use runtime macros and global constants for setting
a custom metric prefix:

    const GraphiteEnv = "icinga.env1"

    object GraphiteWriter "graphite" {
      host_name_template = GraphiteEnv + ".$host.name$"
      service_name_template = GraphiteEnv + ".$host.name$.$service.name$"
    }

fixes #7334