node: Allow OVSSwitch to run in userspace mode

This adds a userSpace parameter to OVSSwitch which tells OVS to run
in userspace mode rather than kernel mode.  From the commandline, this is
--switch=ovsk,userSpace=True.

Note that this makes "ovsk" and the OVSKernelSwitch alias misnomers.  Since
the default behavior is still kernel mode, this is hopefully harmless.