Update enrollment README file
Update enrollment README file

Cleaned up variable list to be a bit more clear to understand
what the variable actually is, as well as added the ironic_url
variable, and noted that the testing variable is defaulted to
false.

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