This change allows customizing the roles required for JSON RPC, to use
any roles different from the 'admin' role.
There is a plan, as part of SRBAC effort, to replace usage of the admin
role in each service by the new service role, which has more restricted
access, and we expect the ironic service user would require only
the service role once this work is done.
Change-Id: I143203be7827f73daba32e7c084c5807063ff91d