Fix bug where both ordinary UserManagementModule and HGUserManagementModules were being added to scenes if no UserManagementModule was specified.

Without explicit config non-hg UMM is used - this is in common with other HG modules.
This was causing a non-HG module to unpredictably use the UMM or HGUMM, though lack of bug reports suggest either UMM was always used or it didn't matter in this case.
Merge branch 'master' of ssh://opensimulator.org/var/git/opensim