CBQE-0: skip test if there is no enough VMs

Change-Id: I2d45376ee3512d2c50172ee77f16eb81ddc29918
Reviewed-on: http://review.couchbase.org/25850
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>
Tested-by: Iryna Mironava <irynamironava@yandex.ru>