slap/standalone: Force remove partitions with chmod

In some cases partitions can chmod -w some paths, so use util.rmtree to
overcome this situation.
6 jobs for fix/rmtree-with-chmod in 0 seconds
Status Job ID Name Coverage
  External
passed SlapOS.Eggs.UnitTest-Jerome.Python2

00:49:52

passed SlapOS.Eggs.UnitTest-Jerome.Python3

00:29:10

passed SlapOS.Eggs.UnitTest-Jerome.Python2

00:38:10

failed SlapOS.Eggs.UnitTest-Jerome.Python2

00:53:43

passed SlapOS.Eggs.UnitTest-Jerome.Python3

00:21:15

passed SlapOS.Eggs.UnitTest-Jerome.Python3

00:30:55