Updates to the ironic-enroll task README
Updates to the ironic-enroll task README

Defined variables and requirements for the ironic-enroll task
README file, as well as provide an example playbook of the task's
use.

openstack/bifrost,bcornec/bifrost,devananda/bifrost,devananda/bifrost,bcornec/bifrost,juliakreger/bifrost,EntropyWorks/bifrost,openstack/bifrost,EntropyWorks/bifrost,juliakreger/bifrost