Fix default Metricbeat config on Windows (elastic#2632)

The load metricset is not supported on Windows, and if it's enabled
in the configuration file, it makes Metricbeat exit immediately with
an error.

This change adjusted the configuration on Windows to exclude the load
metricset.

Fixes 2623.

metricbeat/Makefile

@@ -57,6 +57,11 @@ imports:
 # This is called by the beats packer before building starts
 .PHONY: before-build
 before-build:
+	# disable the system/load metricset on windows
+	sed -i.bk 's/- load/#- load/' $(PREFIX)/metricbeat-win.yml
+	rm $(PREFIX)/metricbeat-win.yml.bk
+	sed -i.bk 's/- load/#- load/' $(PREFIX)/metricbeat-win.full.yml
+	rm $(PREFIX)/metricbeat-win.full.yml.bk
 
 # Runs all collection steps and updates afterwards
 .PHONY: collect