core: refactor CommonVmPoolWithVmsCommand

- Extract the code that add VMs to the pool to separate method
- Rename addVmsSucceded to addVmsSucceeded
- Remove the variable that reflected whether at least one VM creation
  failed as we can know that according to the addVmsSucceeded field
- Rename getAddVmsSucceeded to isAddVmsSucceeded

Change-Id: If9cc7214071e0b92aaa65480f0c5c801538d2c50
Signed-off-by: Arik Hadas <ahadas@redhat.com>