CLOUDSTACK-6323: Change getUser API arg to userapikey

By default the getUser API picks apikey arg from HTTP request to return the
User object with that api key. We use apikey in the request, so it would return
the same user calling the API. This commit uses the USER_API_KEY in the API
annotation instead.

Signed-off-by: Rohit Yadav <rohit.yadav@shapeblue.com>