Added Portainer Agent

[Willem-Dekker]

Added Portainer Agent this allows you to connect your stack to a other Portainer instance/ server

[gcgarner]

Hi Willem

Thanks for the PR. Could you please send me a link to the setup process for the agent so I can make a wiki entry

[Willem-Dekker]

https://portainer.readthedocs.io/en/stable/agent.html

[Slyke]

I have submitted this PR on your behalf over at the fork: https://github.com/SensorsIot/IOTstack/pull/2, If you would like to have your name to the commits, please submit the PR there and let me know so I can remove the PR I created on your behalf.