Add cluster example playbook

[malcyon]

Adding an example playbook to demonstrate a cluster install.

Testing the playbook

virtualenv venv
source venv/bin/activate
python3 -m pip install -U pip
pip3 install -r requirements.txt
molecule create --scenario-name=example2
molecule converge --scenario-name=example2
molecule list
molecule login --scenario-name=example2 -h graylog01

Fixes #89

Reference:

• https://docs.graylog.org/v1/docs/multinode-setup