Detecting an unknown tenant got tricky when we started returning a
message about tenants not being ready yet. In order to be able to return
a 404 for tenants we legitimately do not know anything about, keep an
UnparsedAbideConfig on the scheduler so that we can check it in case of
a miss. If we know about a tenant but don't have a config, we can return
the 'not ready' message. If we don't know about it at all, we can throw
the 404.
Also, remove the custom handler test. We have tests in other contexts
(like tests of the github webhook) that test the equivilent
functionality.
Change-Id: Icff5d7036b6a237646ad7482103f7b487621bac0
9db66082d3