Modify plugins.rst
Modify plugins.rst

* Be clear on what is the minimum for defining a plugin
* How to access options from plabooks, roles
* What plugin types exist

Change-Id: I3d557e2420deb399e512852ee055fefc7e285135

redhat-openstack/infrared,okolisny/InfraRed,redhat-openstack/infrared,okolisny/InfraRed,okolisny/InfraRed,redhat-openstack/infrared