app: fix nonsensical return value for #use_deployment

Since it registers multiple deployments, returning a single model
makes no sense. Instead, return the array of all the resolved
models.
app: fix mixup in the loaders

The first thing this commit does is making sure that the code
in syskit does not access Orocos.default_loader. Since orocos.rb
is currently (mostly) only accessing Orocos.default_loader, they
are identical for now though.

The second thing is that all model loading operations call
app.default_loader instead of accessing the process server's loaders
directly. They are multiple reasons for this change:
 - calling the process server loaders breaks orogen's loader
   hierarchical structure, as models get registered only on the
   root loader by design. The reason for this constraint is that
   a loader hierarchy represents a complete system, in which all
   models should be coherent, and in which a given model should
   be represented only by a single object.
 - having to specify the process server on using_task_library
   seriously hinders reusability, as it sets it "in stone".