Add mutual TLS support to prometheus_client output plugin

Signed-off-by: Robert Sullivan <rsullivan@pivotal.io>

plugins/outputs/prometheus_client/scripts/generate_certs.sh

@@ -0,0 +1,17 @@
+#!/bin/bash -e
+
+scripts_dir=$(cd $(dirname $0) && pwd)
+
+mkdir -p ${scripts_dir}/../assets
+assets_dir=$(cd ${scripts_dir}/../assets && pwd)
+
+echo "Generating certs into ${assets_dir}"
+
+test ! `which certstrap` && go get -u -v github.com/square/certstrap
+
+rm -f ${assets_dir}/*
+
+# CA to distribute to loggregator certs
+certstrap --depot-path ${assets_dir} init --passphrase '' --common-name telegrafCA --expires "25 years"
+certstrap --depot-path ${assets_dir} request-cert --passphrase '' --common-name telegraf
+certstrap --depot-path ${assets_dir} sign telegraf --CA telegrafCA --expires "25 years"