proper errors when there's no service auth token

When provisioning a service instance, raise when no service auth token
is in the db for that service. (We previously give a nil and we would
end up with an unreadable NoMethodError).
Added an auth token by default on service blueprint.
[#44044997]

Change-Id: I397f0cbfb1d49f7532e133d88bc8e3a520620302