beaker cannot find a valid system if a loanee schedules a job against a system which has groups of which they are not a member.

This is because the _available() in model.py does not take into account loaned users anymore. 
The fix is to do 

index 8717939..4240fa5 100644
--- a/Server/bkr/server/model.py
+++ b/Server/bkr/server/model.py
@@ -1721,6 +1721,7 @@ def _available(self, user, system_status=None, systems=None):
  
         if not user.is_admin():
             query = query.filter(or_(and_(System.owner==user), 
+                                    System.loaned == user,
                                     and_(System.shared==True, 
                                          System.groups==None,
                                         ),