Plugin test setup

I've refreshed some of the documentation around plugin testing on
Jenkins for plugins under theforeman.

Follow the "Adding a new …" sections if you're maintaining either a
Foreman or smart proxy plugin to test the main branches. The same bit
then goes on to describe how to set up PR testing for the plugin.

The configs support testing plugin branches against a corresponding
older core branch, so take a look at the foreman_discovery config if you
want to expand your branch testing.

The following plugins have issues that stop me adding them right now:

  1. foreman_host_rundeck:
  2. smart_proxy_chef: Bug #10685: smart_proxy_chef tests: undefined method `new' for ChefPlugin::Authentication:Module - Chef - Foreman

Others are mostly just missing any tests of their own or I'm unsure if
core tests will still pass with them.

··· -- Dominic Cleal Red Hat Engineering