mgr: fix subuser creation via dashboard

mgr: fix subuser creation via dashboard

Subusers couldn't be created through the dashboard, because the get call was overwritten with Python magic due to it being the function under the HTTP call.
The get function was therefore split into an "external" and "internal" function, whereas one
can be used by functions without triggering the magic. Since the user object was then returned correctly, json.loads could be removed.

Signed-off-by: Hannes Baum <hannes.baum@cloudandheat.com>
(cherry picked from commit 90e221d0b53ad137e912b8cbd84935a8755f1fe7)