Batch startup support for OVS

Currently, every ovs-vsctl command requires reading the entire OVS
configuration database. This means that its performance gets linearly
slower as more switches and ports are added. To mitigate this, we
batch multiple configuration operations into individual, long,
ovs-vsctl commands.

This patch set makes a couple of other notable changes, including
setting printPid=False by default (avoids using mnexec unnecessarily)
and running certain commands using errRun rather than quietRun.
Additionally we no longer look for leftover links in the root
namespace, so code relying on that functionality may have to change
slightly (as in controlnet.py and sshd.py for example.) It also adds
cluster support to mn -c.

The performance result is that mn --topo linear,200 --test none now
completes in 60 seconds rather than 95 seconds (on my laptop) without
the patch (vs. 101 seconds in 2.2.0).

This is still slower than I would like - we should be able to make
some additional improvements.