slapos.cookbook: default software_type is now 'default'

Add backward compatibility in switchsoftwaretype recipe for old default software_type (RootSoftwareInstance)

See merge request !1642
3 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed ERP5.StandaloneDeploymentScriptTest-Debian.Buster

00:46:21

passed SlapOS.Eggs.UnitTest-Master.Python2

00:36:31

passed SlapOS.Eggs.UnitTest-Master.Python3

00:40:03