Ansible openstack_full_argument_spec defines the 'auth' param
as dict.
As the enroll task defaults the 'auth' variable to None, Ansible
first sets that to string 'None' which makes it unconvertible to
dict.
Removing the line to leverage the default(omit) fixes the issue.
Change-Id: Ib442fdca64e375eb8025212067032c93934d5d94
Closes-Bug: LP1612196