Create a true Python list of these objects

Create a true Python list to ensure that you only query the database once to generate the public key of the election, without the need for further queries.

I would like to register the other members who helped to identify this issue:
 - Thiago Alexandre Herek - UTFPR / Brazil
 - José Gonçalves De Oliveira Júnior - UTFPR / Brazil
 - Joilson Alves Júnior - UTFPR / Brazil
 - Paulo Matias - UFSCar / Brazil (@thotypous)

helios/models.py

@@ -510,7 +510,7 @@ def freeze(self):
     self.set_eligibility()
 
     # public key for trustees
-    trustees = Trustee.get_by_election(self)
+    trustees = list(Trustee.get_by_election(self))
     combined_pk = trustees[0].public_key
     for t in trustees[1:]:
       combined_pk = combined_pk * t.public_key
@@ -1171,7 +1171,7 @@ def save(self, *args, **kwargs):
 
   @classmethod
   def get_by_election(cls, election):
-    return cls.objects.filter(election = election).order_by('id')
+    return cls.objects.filter(election = election)
 
   @classmethod
   def get_by_uuid(cls, uuid):