switch host key checking off by default, to minimise the amount of configuration needed.

Automatically tested XT machines typically get installed with freshly generated host keys all the time, which
is detected by openssh's host key caching mechanism. Therefore, it is necessary to work around that problem to
avoid errors blocking testing. Previously I have encouraged users to configure their openssh client to
avoid host key checking, but now I'm working to reduce the amount of site configuration needed I think the best
default is to disable host key checking. This means BVT tests will be vulnerable to attackers able to generate
traffic on the networks in which it operates, but in controlled lab conditions this is reasonable.

Running in this mode also revealed some entry points to run.py which didn't have control of host key checking, so
I have extended the mechanism in places.

An approach I have not explored is to override the host key generation on platforms and VMs to make sure that
host keys are reused after installation.
placate pylint
use new PXE infrastructure generation code