archive install check if user and/or service was already defined

[hurrycaine]

At my work we are very specific with how we create our users and groups (uid, home dir etc...).
I also did not like the service that is default with archive install. Chances are someone would create a start up script or something here.

At first I was going to add a manage user/service parameter but since this supports many different types of install methods I thought this would get confusing. Also, I am sensitive to parameter growth.

I have test and have a pull request with a solution. No impact by default. The user and the service under archive only... check to see if they are defined first. This allows someone to create the user prior to call grafana class. The service is a bit tricker as you want it after the install/config but before the grafana::service.
This does not cover the scenario of wanting to create a different user then grafana but I feel this would be pretty fringe.

I have included an advance usage in readme to go over this scenario with examples.

Please let me know if you have any questions