[Fixes #51] Bug: creating user via json_content expects username, also fixes bug when using delete user

[mwallschlaeger]

Description

function expected to always receive a username, but if user gets created via json object username is contained inside json so no additional username must be defined. #51

Further deleting users was using the geonodeobject.py implementation which wasn't accurate for users, added a users specific impl of that.

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closes #51

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