slapos_pdm: Support Hosting Subscription with many root Software Instances

Since during maintenance of Hosting Subscription it's possible to end up with
many root Software Instances (of which many might be destroy_requested) it's
needed to support such situation while creating upgrade decision and finding
upgradeable Software Release.
13 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
failed SlapOS-Master.UnitTest-Master

01:08:59

passed SlapOS.Eggs.UnitTest-Master.Python2

00:36:38

passed SlapOS.Eggs.UnitTest-Master.Python3

00:23:30

passed SlapOS.Eggs.UnitTest-Master.Python2

00:25:17

passed SlapOS.Eggs.UnitTest-Master.Python2

00:09:22

passed SlapOS.Eggs.UnitTest-Master.Python2

00:08:50

passed SlapOS.Eggs.UnitTest-Master.Python2

00:08:10

passed SlapOS.Eggs.UnitTest-Master.Python2

00:08:57

failed SlapOS.Eggs.UnitTest-Master.Python3

00:17:17

failed SlapOS.Eggs.UnitTest-Master.Python3

00:14:31

passed SlapOS.Eggs.UnitTest-Master.Python3

00:13:39

passed SlapOS.Eggs.UnitTest-Master.Python3

00:10:00

passed SlapOS.Eggs.UnitTest-Master.Python3

00:19:04