ovsdbmonitor: Use ovs.json module instead of JsonReader and JsonWriter.

ovsdbmonitor: Use ovs.json module instead of JsonReader and JsonWriter.

I can't figure out where JsonReader and JsonWriter come from.  I know that
they must exist, because I (and others) have used ovsdbmonitor before, but
I can't find them now.

Switch to using ovs.json, which is part of Open vSwitch so we know that
it exists.  At the same time, we have to start translating the Unicode
strings that ovs.json outputs into standard Python strings; otherwise
the "twisted conch" ssh implementation craps out because it tries to
concatenate this Unicode string with a standard string that contains
non-ASCII characters.