We also clean things up a bit and check for error messages,
which now can cause exceptions which should invoke cleanup.

Note that we are changing the interface of batchStartup/Shutdown
slightly so that the method can choose not to start some of the
switches. We might wish to refine this a bit...

Eventually we should implement true batch shutdown.
In the mean time, we just ignore it. Note there's no good
way that I know of for a subclass to remove a superclass
method, so we changed the protocol a bit to require a return
value of True.

use ControlPersist ssh option to create a ControlMaster connection that will not die when a node dies

Note that we do still provide a default if you specify
ControlPath=True

The hope is that this will make the results a bit more consistent
when running in a VM environment.