ofproto-dpif: Use a single underlying datapath across multiple bridges.

ofproto-dpif: Use a single underlying datapath across multiple bridges.

This commit switches to using a single backing datapath (called
"ovs-datapath") for all bridges of that datapath's type.  Previously,
resources couldn't be shared across bridges, since each was in its own
datapath.  This change will allow sharing of tunnels and cheaper patch
ports to be added in the future.

Since bridges share a common datapath, the ovs-dpctl commands won't
provide bridge-specific information.  Users wishing to have that
information should use the new "ovs-appctl dpif/*" commands as
documented in ovs-vswitchd(8).

Signed-off-by: Justin Pettit <jpettit@nicira.com>