node: Allow OVSSwitch to run in userspace mode

This adds a datapath parameter to OVSSwitch which allows one to tell OVS to
run in userspace mode rather than kernel mode.  From the commandline, this
is --switch=ovsk,datapath=user.

Note that this makes "ovsk" and the OVSKernelSwitch alias misnomers.  Since
the default behavior is still kernel mode, this is hopefully harmless.

This is the second version of this patch, which changes the argument name
and values according to Bob's suggestion.