cli: expose --only-cp in slapos node promise

The logic was here, but the argument was not registered
3 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
failed SlapOS-Master.UnitTest-Master

02:23:01

passed SlapOS.Eggs.UnitTest-Master.Python2

00:13:17

passed SlapOS.Eggs.UnitTest-Master.Python3