Update list_replication_targets

Update list_replication_targets

We were planning to let the list targets call be a
free for all, and just be a generic dict of info.

This probably isn't such a great idea, inparticular since
we have a required identifier field for each replication
device it makes more sense to just use that in the response
and leave the rest of the info internal.

In the future if there's a need we can always implement a
show command that displays all the dirty details.

This patch changes the identifier name to 'target_device_id'
to be more self describing, and updates the docstrings and
devref docs to reflect the changes and agreed upon changes.

Change-Id: If14130f5d5bb2e6df4478bd0e14a1f33d706bf78