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.

Background: the reference controller is reactive and installs exact
match rules. By attempting to start a telnet session we make sure that
the ARP caches and TCP flow rules are set up (in one direction at
least) before the test starts.

This is intended to help with #413

add 1ms delay to all links to exaggerate TCP bandwidth decrease across an increasing number of links