The problem was that the first call to checkMetaOffline() throws an exception
which results in image_repo_->checkMetaOffline() not being called.

There are a bunch of root causes that should be fixed at some point:
 - The *Repository classes are not fully initialized in the ctor
 - The required initialization is called 'checkMetaOffline()''
 - There is no documentation that says this is so
 - There are no asserts to enforce the above requirement at runtime.

There only one place that uses this default, so remove the default ctor and
explicitly express it in RepositoryCommon. This means that no-one has to
remember "What is the policy for a default-constructed Root object?"" any more.

This wasn't implicated in TOR-3452, but the new version is shorter and has
simple control flow.