Make the default mandatory powerdown period configurable per PDU

The executor currently waits 30 seconds before re-applying power to a DUT to boot it, but this is not necessary when the DUT is a VM connected to a vPDU.

The executor should use 30 seconds except when the PDU is a vPDU, in that case it can use some really short period (0.1s?). This would speed up the integration tests in CI by around 2 minutes, since the DUT is currently powered off/on 4 times during that testing, with each incurring a separate 30 second wait period