Add supervisory_authority plugin

[laugmanuel]

needs: theforeman/foreman-packaging#3625

[ekohl]

Would it make sense to split this up into Parameters and Advanced Parameters? If not, I prefer the puppet-strings syntax since it's the officially supported one. However, RDoc has the feature of parameter groups which we do use in our installer.

[laugmanuel]

I added the "Advanced Parameters" section with all parameters which are optional.
The doc-block was taken from another plugin (memcache) to be consistent with whats already there.
I think we should rework the RDoc in all the Puppet files at once to not have different styles of documenting stuff.
What do you think?

[laugmanuel]

I did change the default values of some optional/advanced parameters to correspond with the default values of the elastic documentation.

[ekohl]

Thanks!