executor: set the default min off time based on the PDU
This will enable MaRS to use PDU-provided defaults, rather than a hard-coded 30s for all PDUs.
Honestly, this is only useful in the case of the vPDU, as we do not need to wait for any capacitor to discharge before being able to turn the machine back on again.
With this change, vPDU-based hosts will have a 0.1s off time rather than defaulting to 30 seconds. This decreases the execution time of the integration test by 2.5 minutes, out 11.5 minutes!
Fixes: #132 (closed)