restapi: Use DbUser.isGroup() properly

Replaced an explicit string check on DbUser's user name with the BE's
method isGroup().

This achieves two goals:
1. It's easier to understand.
2. It removes a dependency on the deprecated StringHelper.

Change-Id: Ib17d50003d2b1425db9a2fc17c206da2d4db5de7
Signed-off-by: Allon Mureinik <amureini@redhat.com>