Convert DefaultableCapabilityReferences to standard CapabilityReferences

The intention of the DefaultableCapabilityReference is for attributes that should reference one capability when defined and another when undefined.
e.g.
/subsystem=infinispan/cache-container=*/transport=jgroups contains a channel attribute that references a named channel when defined, and the default channel when undefined.

The current resource description (as returned via read-resource-description operation) does not fully describe this behavior, as it assumes an 1:1 association between requirement and attribute.

We can safely replace this with a resource-level capability reference on the default channel plus a standard capability reference on the channel attribute.