tests: Move VM starting delay control into parallel-vm.py

tests: Move VM starting delay control into parallel-vm.py

This is more efficient since we can now start only the necessary number
of VMs instead of always forcing all VMs to start with one second delay.
This can also control the starting delay by keeping at most two VMs
starting at a time instead of using the hardcoded one second wait for
each consecutive VM.

Signed-off-by: Jouni Malinen <j@w1.fi>