Add workaround for Heartbeat install issue

[dedemorton]

Documents the workaround for #3765

@geekpete Can you test and confirm that this is accurate?

Here's what the change looks like rendered in the Heartbeat docs:

[geekpete]

I'll test this on these two OSes shortly and let you know.

[geekpete]

LGTM

Using Docker tested Ubuntu latest, Ubuntu 16.04.2 LTS, Debian latest and Debian 8.7

docker run -it -l ubuntu-test ubuntu:latest bash
apt update
apt install -y wget sudo
wget -qO - https://artifacts.elastic.co/GPG-KEY-elasticsearch | sudo apt-key add -
sudo apt-get install -y apt-transport-https
echo "deb https://artifacts.elastic.co/packages/5.x/apt stable main" | sudo tee -a /etc/apt/sources.list.d/elastic-5.x.list
echo -e "Package: heartbeat\nPin: origin artifacts.elastic.co\nPin-Priority: 700\n" > /etc/apt/preferences.d/heartbeat
sudo apt update && sudo apt install heartbeat

Other images used:

docker run -it -l ubuntu-test ubuntu:16.04 bash
docker run -it -l debian-test debian:latest bash
docker run -it -l debian-test debian:8.7 bash

Also tested upgrade (just on ubuntu latest) from heartbeat 5.2.1 to 5.3.0 and that worked as expected also.

[andrewkroh]

LGTM. Is this still "in progress"?

[dedemorton]

@andrewkroh I removed the in progress label. This one is ready to go.