Check for endpoints in response, displaying error message properly
Root containers are droppable.
Merge pull request #556 from jaycee/drag-and-drop-mv-reloads-page

Root containers are droppable.
Merge pull request #555 from makyo/add-subrel

Display an error if Juju will not allow a subordinate relation.

Check for endpoints in response, displaying error message properly; fixes 1369606
Make the GUI aware of the new Subordinate info.

After a recent change in juju-core, the subordinate
info is available through the mega-watcher for
services and units. This branch enables the GUI to
fully support this where possible.

Also fixed the code so that uses of "subordinate" vs
"is_subordinate" are consistent.