[RFE] devstack plugin should use the "service" user instead of "admin"

In ironic/devstack/lib/ironic create_ironic_accounts() we currently create the ironic user with the "admin" role:
  create_service_user "ironic" "admin"

I don't believe that is necessary - and we should, instead, create the ironic user with the default "service" role.

However, I am unsure as to whether this will complicate our interactions with Neutron ports when attaching/detaching a Port to a tenant's network, and it's unclear how much work that will require.