slapos_payzen: Allow projects to custom the expected html

See merge request !498
5 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
failed SlapOS-Master.UnitTest-Master

03:46:14

passed SlapOS.Eggs.UnitTest-Master.Python2

00:29:35

failed SlapOS-Master.UnitTest-Master

02:22:15

passed SlapOS.Eggs.UnitTest-Master.Python2

00:14:10

passed SlapOS.Eggs.UnitTest-Master.Python2

00:15:40