Securitas Flow: Use reservation email address

When creating the access grants for Securitas Flow, user the
email address from the reservation first and fall back to the
user's email address if the reservation doesn't have the reserver
email address specified (i.e. it's not asked in the reservation
form).

Refs TTVA-214

kulkunen/drivers/securitas.py

@@ -43,6 +43,7 @@ def install_grant(self, grant):
         assert grant.state == grant.INSTALLING
 
         user = grant.reservation.user
+        email = grant.reservation.reserver_email_address or user.email
 
         user_id = str(user.pk)
 
@@ -65,7 +66,7 @@ def install_grant(self, grant):
                 "userExtId": user_id,
                 "firstName": user.first_name,
                 "lastName": user.last_name,
-                "email": user.email,
+                "email": email,
                 "notifyUser": True,
                 **params,
             },